Replacing mid-80s Denon amp and preamp with new Parasound equipment

Assuming you'll keep the Polk loudspeakers, then keep the Denons.  The Parasounds are good quality equipment, but it's not a big step up from the Denons in your system.  I would have the Denons tested to make sure they are operating properly.  They may need to be re-capped or cleaned.

If you do go the new amp route, the SDA speakers need a common ground amp.  I'd do some reading on this if you're not familiar with the topic, I'm not actually sure what the design is.

Not certain overall of the Denon over the Parasound except definitely better low impedance driving capabilities.  I had the 2125 for a while and it was quite the "budget" amp with loads of juice. 

I had the Denon PRA-1500/POA-1500 combo driving a pair of Klipsch fortes and the combination was incredible.  Fast, punchy, great detailed midrange, and a very open sound.  Tiny turn of the volume knob and the fortes were blasting beautiful muisc all day long with no end in sight.  On a whim I hooked the same Denon combo up to my Klipsch RF-63's and twisted the volume to about the same spot and after about 15 minutes the Denon amp was getting hotter and tripped the protection circuitry.

Had it repaired by replacing two pairs of output transistors and back to business again playing the fortes.  No more sub 4ohm loads from the RF-63s.

I read in a few articles that the POA-1500 was a wonderful amp as long as you don't present a sustained sub 8ohm load and expect it to play at modest to loud volumes.

Bill