Which is best sub for N802 in fairly large room?

Theil has come out with a moster of a sub that sounded quick articulate and alot like a closed box design at the dealers. REL makes some pretty good stuff, but I'd go with one that has some type of parametric eq filter to take care of the room gain that it sounds like you hearing, and a monster of an amp to go with it. Something over 800 watts.

I have the N802s and use a Velodyne HGS 15 with good results for movies.
If it is music you are going for, I would look at upgrading the PA7 first with a newer amp to improve on the overall performance of your system. Next would be a DAC (the Bel Canto DAC 2 is great value and will be a significant improvement over the Sony internal DACs...) My Kora usually outshines the SACD (Sony SCD 777ES) recordings with Red Book CDs. Last of all the cables will also make a big difference.
I use a pair of Belles 350A monos with the Belles 21A pre CD is a Sony SCD 777ES (SACD) / Kora Hermes 24/192 DAC.
The bass is excellent for music without a sub.

Benefits of a better amp will blossom with cleaner mids / smoother highs as well.

I recently heard a SIM P5 / W5 combo that was very impressive driving N801s. In the same price range as a good Velodyne ($1500 - $2500) you can get a used high quality amp or perhaps 2 for Bi-amping.
I would look at Simaudio, Classe, Belles and McCormack to name a few.
Keep in mind as you go up this ladder, speaker positioning, quality cables (NBS, Siltech, TARA Labs Air series, etc.) and room accoustics / treatments are as important as the electronics and quality speakers.

A new sub will give you deeper bass but it will not clean up the system's overall sound and provide more musical sound like newer electronics & cables will.

At the end of the day it is what you like, what you want to spend and all of us have our own views.

Don

You might want to try a lower crossover point, and a better crossover like the Marchand. Take a look at my system, I am using the Marchand and two 12" subs and love it.