I don't know about the design, but I have recently been looking at a lot of polk stuff, and i made a thread on this forum about it, as well as checked a couple of other threads here.
Polk is a widely respected brand from what I can tell in the past couple of month's research. Asking around here, it seems like you're either going to get someone saying "go with what sounds good to you," or someone saying basically polk sucks (there was this one guy "Send Margaritas" who just bashes polk pretty much arbitrarily because, from what I can tell, they are too mainstream for him... only reason i mention it is because he took a lot of time to do so).
I am in no way an audiopohile or a huge techy when it comes to speakers, but I listen to music CONSTANTlY and so I care a lot about my speakers. I had RTiA3's and was not thoroughly impressed with them. But then again they were 400 bux MSRP for the pair, which really isn't that expensive, and I had quite possibly the worst room set-up this forum has seen, as well as the left channel on my receiver making white noise.
I can only tell you a little bit about my experience with those.
The first thing was that their break in made a LARGE difference. It was noticeable. I'm not an audiophile, but to me it sounded like the midrange cleared up and there was more responsive low end.
So sorry for just making generalizations.
Basically, my advice is to go look at some serious reviews of each speaker. After I have done so, I can say that polk is a solid brand in MOST departments (definitely not flawless), and respected even among some of the most respected reviewers. Among the "tweak" community, i think they get a lot of hate for being more mainstream.
From what I have heard about the RTiA's in general is that they are more of a powerhouse speaker (as opposed to the detailed LSi's, which after reading MANY MANY reviews and posting here, I have decided to buy).
PSB is a great brand, as well, from only what I have read (haven't hear any, yet).
In the end, I highly doubt you will be unhappy with either.
Also, what kind of jazz do you listen to, because if vocals are something you love in music, this should affect your speaker choice. A lot of reviews of the LSi's I read were particularly impressed with the speakers' performance on vocals. IDK if this holds true throughout polk. In my opinion, Jazz is best listenned to on something on the warmer side. Especially if you listen to a lot of much older recordings. This is just from my experience transcribing and listening. The ride cymbal especially can get a little taxing on the ears at times.
Also, you mentioned prog rock. I used to listen to a LOT of yes. Especially Close to the Edge. I prefered that on a warmer system. Steve Howe's tone, especially, was taxing on my ears on the brighter stuff I hear at friends houses, though I loved him so.
Again, I'm no expert, but I thought I'd try to help. I hope the experts refrain from giving me a whole bunch of crap. (last time I asked if I should get the LSi's, and some dude tried to psychoanalyze me)
