The receiver isn't making it sound "dark/compressed/dull", that's far from the realm of possibility.
Those are pretty dainty speakers. I can't imagine they are very dynamically capable given their size and orientation. Those speakers just will not be able to deliver the crushing output it sounds like you want either.
If you describe the sound you are hearing to dark, compressed, and dull, I'd recommend checking out Klipsch. Many of their models are quite dynamic, they aren't dark, and they definitely aren't dull. I personally like the way the SF-2 floor standers sound for their lower end models and they can go for pretty reasonable prices in the used market. They are also pretty easy to drive and will go significantly louder than the PSb Alpha B1 speakers given the same wattage input.
I picked up on that you where considering room treatment as a possible solution to your sound quandry. Acoustic treatment would have a profound affect on your sound accuracy. Reading your description of how it sounds makes me pretty sure it's not the culprit of what you aren't liking here, unless your speakers are just plain set up wrong. I think you can skip this step for now.
A cool factoid about the SA-XR57: it's tested power is exceedingly high for a 10 pound receiver. It uses a derivitive of Class D power, a switching amplifier, and works very well with complex loads given it's size. The Denon AVR-2803 seems to do the opposite of what I'm used to seeing a Denon do into 4 ohm loads, and that's lose power. It's ACD performance is also pretty poor, but all channels driven has been found to be less important in virtually any scenario as it's rarely called for. I do think the AVR-2803 is fine for now, but if you can pick up a SA-XR57 for a song it's worth a shot.
