You would need a separate preamp, phono stage and DAC unless you got one of the varieties of preamps available that have a built-in DAC. Then you might still need the phono stage, as I don't recollect any preamp/DAC having one.

I have a Preamp and amp that is to die for in my big system; I have been listening to audio gear for 30 years in the quest for

the ultimate sound, and I think I pretty much have it now. I use an Audio Research LS-26 preamp and a Bryston 3B-SST power amplifier.

That is a cost of around $10,000. The current model number of the preamp is LS-27. Nothing obvious has changed, but there must be some some changes of a minor sort.

In my AV system at my other house, I use a Musical Fidelity M3i, which is very very good.

I would hesitate to say it is better than the Rega, though, because from what I hear about it I suspect it is in the same class.

In my opinion you would have to spend at least $5000 to get much of an upgrade from your present amplifier if you get a preamp and power amp.

In your situation, I would get the Musical Fidelity V-LPS phono stage for $150, and the Vincent SP-331 power amplifier for $1300.

I would definitely try using the Mira as my main preamp and see how that sounds; it has preamp outputs that you can run to the Vincent amp. The Mira preamp stage may be good enough that mating it with the Vincent power amplifier may be a huge upgrade in your sound. The Vincent won "Component of the Year" a while back and it is just fabulous for its price; you will almost certainly love it, and it will drive those Mags for sure with 300 watts P.C. at 4 ohms. That is my guess, anyway.

Add the Musical Fidelity V-DAC Mark II for $349, and you would be in business for well under $2000. Is that in your budget?

If you still want a big improvement in sound quality after that, I would get the Audio Research LS-17 preamp for $3000.

ONe thing I definitely recommend, however, is the Musical Fidelity V-LPS phono preamp, which is incredibly good desite being very inexpensive. In my big system i have the Audio Research PH-5 phono stage, which cost me $1500 or so, and the V-LPS in my other system is almost as good!! The AR has a lot more bells and whistles, but the V-LPS works with any cartridge and is excellent, for only $150 or so.

at $6,000 it is probably a nice piece.

Gram Amp 2 SE phono stage for $410 would be pretty nice. There would be no problem driving those Maggies with that!

http://www.peachtreeaudio.com/grand-integrated.html

Perhaps you've already gone too far beyond that, but in response to your original post: the most expensive model in the well-reviewed Benchmark line of DACs includes adequate switching for your needs, but I'm pretty sure it doesn't handle any of the digital outputs of the iMac.  Being a PC guy myself, I can't say what you'd need to adapt the one thing to the other.  But modulo that question, it represents a $2K solution to the DAC-plus-linestage problem.  As to the phono preamp, I'm happily using a $400 Graham Slee myself (with a MM cartridge), but the MF V-LPS didn't exist when I bought it, so I can't compare the two.

The idea of putting all of your eggs in one basket is a bad idea from my perspective. A unit like that is going to

be a technician's nightmare to service, as i have personally experienced, plus anything that goes wrong with one section 

means the whole thing must be shipped away.

I would still recommend getting a good power amplifier and using the existing unit as a preamp.

A separate DAC is always best IMO to keep the digital circuits in their own box with their own power supplies; much to be preferred! It is stupid, from an engineering standpoint, to put all of those digital nasties in the same box with the analog circuits and then have to go to all sorts of lengths to shield them from the rest of the unit.

A separate phono preamp is also best since it is dealing with very low-level signals and needs to be completely isolated in its own chassis for best signal-to-noise ratio. These things are kept separate for good reasons! Again, the Musical Fidelity V-LPS is excellent and quite inexpensive. It is also small enough to only take up a small space; probably next to the turntable.

If you want to "head-bang at top volume", you had better be careful with Magneplanars! They just can't play at really high volumes without damage. Go too loud ONCE and you will be sending them to the factory for expensive repairs.

The basic structure of their transducers makes them dynamically limited; they DO NOT like to be driven too hard and can be easily damaged by being overdriven. The transducer elements have a definite physical excursion limit that you should not exceeed.

The reason that larger amplifiers do better with them is not because of the high power per se (which they don't really need), it is because the Magneplanars work best with an amplifier that has a very low output impedance (less than 0.3 ohm). The amplifiers with the lowest output impedance usually ARE hgher-power solid-state amplifiers, although even that is not absolutely 100% true.

The typical output impedance from the "4 ohm" output terminals of a 20-80 watt tube amplifer is on the order of 1.2 to 2.8 ohms, which is not appropriate for best performance with Magneplanars; not good at all.

Some music can sound really wonderful with maggies and a small tube amplifier, but only small-scale music that puts limited demands on the amplifier; other music may not sound good at all.

If you want to drive them with a tube amplifier, keep in mind that all but the most powerful/most expensive tube amplifiers have relatively high output impedance and limited bass power; these are not characteristics that match up well with the Magneplanars.

I would advise a tube amplifier that is at least 80 watts per channel if you want to try it. Most tube amplifiers have lower power simply because it costs a LOT of money to make a higher-power tube amplifier. It takes a VERY large very expensive output transformer and a very expensive power supply section to supply it.

The Musical Fidelity M3i integrated amplifier is one that might be a good choice to drive Magneplanars. It puts out around 110 watts at 4 ohms and has an output impedance of 0.25 ohms, according to tests done at an independent lab for an Australian hi-fi magazine. It is a very good-sounding amplifier that should match them well.