Polk makes some very good speakers; they are generally quite well-designed and offer good sound for the price. I see that Amazon is offering the Monitor 70s for $200 each with free shipping, which is half of the list price; quite a good deal, it would seem.

While I have never heard your current speakers, my experience with Radio Shack speakers and Polk speakers leads me to think these are likely to sound a lot better than what you have now. If $400 is what you have to spend, this might well be the best deal available.

I bought the Polk RTA12 Monitor speakers when they first came out in 1980; that was their flagship speaker then, selling for $1200. They were far better than the rest of the equipment I owned then, so it took a while for my electronics to catch up and do them justice.

The PSB Image T6 is a really excellent tower speaker, which I am currently using in my system at my second home with a Musical Fidelity M3i amplifier (a marvelous-sounding amp!!), and somewhat similar in size and design, but they cost $1300/pair.

P.S.- You may want to check AudioReview.com where there are 5 or 6 reviews posted by owners of the 70s.

Another thought; For a bit more money...The Monitor Audio Bronze BX-5, at around $650, might be a good choice. I suspect that they would be better-sounding than the Polks...but that is more moola...lol. 

Right now I own a pair of old Roger Sound Lab bookshelf speakers. I like the speakers but want to move up a little. Real interested in two: Polk Monitor 70 and Polk LSi9. My receiver is a Denon AVR-1912. I have an Onkyo DVD. I Run a Mac Mini through the Denon. My center and rear speakers are KEF as is my subwoofer (2150). I am not an audiophile per se, but I like great sound in music and movie and streaming. While I like the floor standing, not sure my wife will dig it! You get what i am sayin' if you're married. The Polk LSi9's look like they kick ass.

Any advice or direction would be helpful.

they need considerable power from an amp comfortable running into a 4 Ohm load. The Denon AVR-1912 is recommended to only be used with loudspeakers with 6 - 16 Ohm impedence. The Polk LSi9s are by far better than the Polk Monitor 70 although they don't dig quite as deep, but that is what a subwoofer is for. It looks like it might have to be the Polk Monitor 70s for you unless you upgrade your amplification.

However, I would also be inclined to seek a timbral match with your center and surrounds, suggesting front speakers with the same drivers as the rest. Can you get KEFs to match?

Hmm. Thanks for the tip. I have the KEF's that came with the KEF sub, but to me they just aren't robust enough. What would you recommend if the LSi9's and the Denon AVR-1912 don't "gel" ?

LSi9s, especially at the same time as the center and surrounds are being powered. While that AVR has individual amplifiers for each channel, what they fail to mention is there is a single, rather limited power supply providing current to all of them. Hence, if you are wanting a 5.1 setup with the Polk LSi9s as the front speakers, you will likely need something other than the Denon AVR-1912. If that Denon were an HT receiver with preamp outputs the solution would be rather easy; just add a powerful two channel power amplifier. Options include an inexpensive yet competent AV receiver with preamp outputs and a stereo power amplifier, or an AV preprocessor and sufficient amplification to power all your speakers.

Would the Denon work with the Polk's sans rear speakers? I am trying to find a suitable solution to replacing the RSL speakers with some heart pounding bookshelves that the Denon can power sufficiently. Any modern bookshelf speaker advice would be awesome.

Not adding anything to the amplification issue, but a couple of days ago, a friend brought over a pair of Polk LSi9's that he'd bought on a whim from Polk's discount store. Got them for half price. We wanted to compare them to Epos M12.2's.

We just set the LSi9's next to, and a little lower than the Epos, so didn't try to optimize placement of the Polks.  Amplification was the Grace m903/NAD C275. We went back and forth with different type of music for a couple of hours.  In general, the Polks had a fuller, and slightly smoother sound, and in a way, sounded like a bigger speaker than the Epos. The bass seemed to be over emphasized to me, and slightly boomy on some music. The midrange again seemed fuller sounding, but on at least one female vocal, seemed a little too rich. The treble was smoother, and I thought maybe a little lower in level than the Epos.

My friend and I agreed on most points, though not necessarily strongly. He preferred the sound of the Polk's, though he felt they were probably voiced to emphasize the lower end, (which different placement might change). I found the Polks rich sound kind of hid some detail, and while I liked the smooth treble, I wanted a little more of it. The Epos are a brighter sounding speaker in comparison, though the treble can be a little rough at times. The Epos also have good lower bass output in my room, but the very upper bass, and lower midrange is a bit weak, so while they come across as a leaner sound without any bloat,  I sometimes hear a sort of a disconnect between the low bass and midrange. Anyway, I preferred the Epos over the Polks in spite of their bright, and kind of aggressive sound. If the bass of the LSi9 were tamed, and the upper frequencies increased, I'd enjoy them more. 

What I heard from the Polk LSi9 was mostly verfied from this quick and dirty room response chart that we did after the listening session.