Interesting results Lej1447! I have a few questions regarding the test you conducted.
You said after setting the 8.3s to small you adjusted the crossover higher on BOTH the Denon and Polk PSW350.
When setting your speakers to small and allowing the Denon to perform all the bass management, you should BYPASS the crossover in your powered sub by going "direct in" otherwise you'll have a phenomena known as "cascading filters", which is two crossovers one after the other performing the same task near the same crossover point. This should be avoided because it can hamper the performance of the subwoofer and cause strange dips or humps near that area.
One way to avoid this is to either use the "direct in" jack on the powered sub which in most cases completely bypasses its low pass filter, thus allowing the Denon to handle all the bass management with only one low pass filter in the mix. I noticed that the PSW350 has three inputs. Speaker level, LFE and line level. In your case you should be using LFE because your intended use is for movies. I don't know however if this input bypasses Polk's crossover.
If the LFE input doesn't bypass the subs crossover, then crank it as high as it will go so the Denon's and Polk's filters aren't close enough to affect each other. An example would be setting the Denon's crossover to 80Hz and turning the Polk's all the way up to its 160Hz max setting. You'll still have cascading filters, but this configuration wouldn't be as bad as if they were near the same crossover point. When I was using the NHT SA-3 amp/crossover which is intended for use with my NHT SW3P passive subwoofer, this is exactly what I did since the SA-3 doesn't have a direct in jack. Now that I'm using a Bryston 4B instead of the SA-3, I go directly from the amp to the sub without any filters in the path.
Also, you said you turned the crossover on your Denon up. To what frequency setting? If I had your system I would experiment with LFE settings on your Denon starting in the 80Hz range [if adjustable], this will aleviate much of the bass duty your ARs are enduring while retaining the qualities you like. I don't know if it's the same for your unit, but I see that the AVR-2803 has switchable low pass settings of 40, 60, 80, 100, and 120Hz. If you had set your Denon to 120Hz when you did your test, this could explain why Ian McKellan sounded like a little girl. No, not the OTHER reason, heheh! And, if your Denon was set to this while the Polk had a slightly lower setting these two things in combination could cause a "hole" or "suck out" at that frequency, thus the lack of warmth which is usually attributed to an upper bass hump. Also, try the 60Hz setting and see how that works.
Check all these issues pertaining to your LFE settings, because something as simple as this could be at the root of your dilemna.
In the end, it may just be your Denon. After all, its 90wpc rating is probably a bit optimistic and not an indication of its actual ability to supply dynamic power and high current on demanding movie tracks. It's like my old Integra GSR which had a high horsepower rating and could rev like a demon, but was dog at low RPMs because it had no low end torque at all.
Another thing that might complicate your situation as well is speaker AND subwoofer placement. If you haven't taken a lot of care in placement and set up, search on line for some guides for HT speaker placement. You'd be surprised how vital proper placement of all speakers and especially the subwoofer is to achieving a great movie experience. Yes, subwoofer placement is crucial and in my opinion the most difficult speaker to position.
Sean, quite frankly I'm stunned that you SAY you hadn't thought of the "small" setting for Lej1447's 8.3s. Over the years, I have enjoyed and learned immeasurably from your posts pertaining to 2 channel audiophilia. I'm very honored by you compliment!
Lej1447, if you think setting up a home theater is CONFUSING, don't ever take the step to 2 channel listening. I find this arena to be far more complex with endless upgrade paths and limitless permutations in search of musical bliss. But the rewards can be magic!
Keep at it!
