Active crossovers are unnecessary in a small home enviroment. They are used and useful in large areas where multiple speakers are used across an area, to optimize how they ineract with the room and large spaces. In the small closed enviroment of a home, not needed.

Speakers have evolved and improved big time over the ground breaking in it's time AR-1. Same for the double Advent system, I used it, in it's time it was good, but that wasn't enough either I added Dynaco A-25 and AR-2aX and MicroAcoutic tweeters across em powered by Dynaco ST400(kit built of course by ME) It was a smokin system
) IMPACT and SLAM in it's time, nothing like a stack o' speakers in a 10X15 room...I musta' been influenced by a stack of MARSHALLS ) compared to todays stuff, it's obsolete. 30+ years have improved speaker drivers, cabinets, and everything in between. Ribbons baby, RIBBON tweeters. Better materials, it's all better. I suspect if you compare the double Advent system to a modern quality speaker, it wouldn't sound all that good anymore. Like a bi-Plane to a jet, it works, but why bother?

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Are the computer modeling programs for ported speakers that good?

Yes, they've come quiet a long way since the cut and try methods of JBL back when you bought the Advents. The T&S parameters are much better understood and employed in speaker enclosures today. This allows a ported design to actually have a lower system resonance for the same size enclosure than an A.S. system. (Wasn't that the original selling point of the AR line?) Depending on the resonant frequency of the driver, a ported system might have a F3 of 35Hz. A similar driver placed in a sealed enclosure will have a likely F3 of 65Hz. Part of this results from few manufacturers even promoting a driver that is well suited to a sealed enclosure. Martin King among others has developed a fairly simple MathCad program for ported systems which has helped plenty of DIY'ers. http://www.quarter-wave.com/

The proliferation of home theater systems has become the main driving force in speaker design for the majority of the industry. Or, so it seems, at least. Since HT doesn't require accuracy in most applications, a ported design will have more benefit by adding the 3db of extra level over a similarly sized A.S. system. This goes a long way when you're running off the current-limited amplifiers found in most AV receivers and the speakers must fit into the family room. And, as Advent and AR always pointed out, if speakers are compared in a showroom, make certain the two competing models are demonstrated at precisely the same volume. Though almost all the competition are now B.R. systems, the ported design tends to jump a bit more when the novice hears it in a showroom.

Resonance of the cabinet doesn't really enter into the picture as a sealed cabinet should have less resonance than a B.R. since the B.R. is playing "resonance" through the port. It makes for a distracting sound but is unidentifiable to most people as anything other than "more bass".

With a powered subwoofer in a system, the low frequency roll off of a ported system is actually a benefit rather than a curse. With the second order roll off of a sealed system, the main speaker overlaps the sub at too many critical frequencies unless it is rolled off electrically.

A sealed enclosure has one lump in the system impedance, a ported system has two. This shouldn't affect the choice of crossover since the system resonance falls beneath the point where the crossover acts. The choice of crossover frequency and filter slope will determine where the upper frequency roll off of the low frequency driver begins and at what rate it rolls off. I would find it very unusual to find any manufacturer who used a correction network to smooth the system roll off at the lowest frequencies. If a manufacturer wants to employ a boost to the lowest octaves, they will typically resort to active EQ for the benefits of cost and minimal phase shift or have the driver designed to give a bit of a lift in that region. Or, more commonly, design a driver with the classic BBC lump around 110Hz where plenty of midbass energy gives the impression of deeper bass response than really exists. Of course, if accuracy isn't required, changing the Q of the system will give the impression, to many listeners, of more bass power around system resonance.

Keep in mind that any benefit you find in audio is usually offset by at least two negatives. First order filters have less phase shift than other designs, but they are not totaly immune to phase shift. If the designer employs either a capacitor or an inductor to achieve the roll off, the electrical phase angle will be affected. The resulting phase angle will be 90 degrees out of phase between the low frequency and high frequency driver. This doesn't make for the "best" impulse response, which is generally agreed upon as existing in second order filters where the phase shift is 180 degrees out of phase. The first order filter's 90 degrees + or - is the minimum phase shift you can achieve without further complicating the system or resorting to a single driver, crossover-less design. http://sound.westhost.com/site-map.htm

The typical problem with designs such as the Thiels will be the overlap between drivers above and below the crossover frequency. With a -6dB roll off applied to a woofer, if we cross the system at 2,500 Hz, the woofer will still be contributing significant energy at 6-7kHz. More importantly, the tweeter, assuming a two way system, will be asked to operate down to about 800Hz. This is well outside the normal operating range for almost any extremely well designed and constructed high frequency driver. Not only will you have significant breakup and distortion of both drivers within the operating range, but the dispersion of both drivers will shift dramatically at the overlap. The two drivers are unlikely to be made of a similar material so there will be a difference in timbre that affects the overall impression of accuracy with the entire speaker. The drivers will exhibit severe peaks and dips in frequency response at their respective limits of operation and the two drivers will not sum smoothly into a flat response. Finally, the two drivers will have to be very well made and constructed to even begin to mitigate these problems. That means the drivers will have to be even more expensive. Concessions to some of these problems can be made in the design and construction of the drivers, but that further increases the cost. Those are a few reasons many designers shy away from first order filters on all drivers. I would suggest you investigate a speaker where the low frequency driver is first order with a corresponding or steeper mechanical roll off designed into the driver's suspension. Add a tweeter with two cascaded second order filters for the smoothest crossover with the best impulse response. Of course, phase shift is still only a minimal part of what makes a good speaker system. Ask your salesperson about the amount of correction networks employed in the Thiel speakers. This tends to make the Thiels, as a whole, a difficult line of speakers for most amplifiers. Refer to the reviews of Thiel designs in Stereophile to see the amount of current a typical Thiel design requires before deciding whether your amplifier can handle the load. If possible, ask about the total number of components employed in a typical Thiel filter network. If you like the Thiel sound, there really are no other speakers that do what Thiel accomplishes. If you have the amp to drive them.

As to why very few designers use active crossovers in consumer audio, you have me. The problems of driving a passive vs. an active crossover seem to suggest none other than the cheapest speakers should bother with a passive design. But, then again, I don't understand exactly why anyone wants more than one driver in their enclosure, if you must have an enclosure, anyway. I just bought a Nady two way, stereo, active crossover with a subwoofer crossover included for use as a subwoofer LP filter in my HT system. It cost me $59 and while not the most transparent product I own, for a HT system, it gets the job done. I don't know if the complexity of more amplifiers and interconencts flumoxes too many folks, but you would think this would be right up the aisle of the high end crowd who could make a simple system into an even more complicated and costly endeavour.

You can refoam those Advents and still enjoy the memories.

Look for a programmable remote with learning capability. You will have the ease of plug in codes and the ability to learn the basic commands for the off brand stuff that might show up.