B&W 805 Nautilus for PC?

They definitely are not bright.  They sound muffled, almost like there is a pane of glass between me and the speakers.  The louder I turn them up the better they sound but still not great.  I do have them pointed directly at me.

Did you measure the response?

The sonic contribution from electronics is usually quite small compared to speakers. Therefore, changing that is unlikely to change the sound very much (unless you are using your computer’s internal soundcard/DAC - they are often terrible).
However, there are a few things that may help. The first is that the tweeter should be at ear level, and as exactly as possible. I use Harbeth P3ESRs desktop speakers and lifted them a bit from my desk, for proper response. In addition, I was suffering from a boomy bass due to the proximity of the desk, and raising the speakers made quite a difference. Second, if, as I did, you still find they are too boomy/muffled, and if you are using a Windows computer as a source, you may use the Equalizer Apo software to tame the bass. I measured the response with REW, and created an equalization file to reduce some bass peaks. I downloaded that into Equalizer Apo and it did clean up the sound. Good luck.

Are they bi-wireable? Try disconnecting the tweeter section. Does it sound the same? If so, tweets aren't working.

I suspect you should run them like burn in, I don’t think the denon receiver and your cable are the right one for the 805 ,silver are good for those speakers..clear day cables are silver affordable , to get the most out of it....get good receiver...