Hi
@pseals68
I thought I'd throw in my recent experience. In the past month I got the AVC-3700H, and a set of Q Acoustics speakers (no subwoofer yet).
I have experienced a "muddy" sound, and overly strong "boomy" bass. It took me some investigation, but I figured it out.
Muddy Sound:
I had muddy / garbled sound from my rear surrounds, but not always. I tried different settings etc in the AVR, and sources. It came down to two things:
1. The source (streamed) just had bad rear channel feed (garbled sounds).
2. Leaving Dolby Surround setting on when playing back Stereo feed from the TV source. (muddy)
Boomy bass:
On initial set-up (after Audyssey set-up was ran), the bass was really strong at a very particular small frequency range.
1. I moved the tower speakers further away from the TV
2. Tried putting the bungs in the port (made the sound so much worse), so removed them.
3. Downloaded the Audyssey App, and found it set all my speakers to large. I changed the rear and centre to small, and played with the crossover setting. This really fixed it all.
Anyhow, hopefully there is something here you can try to help your case.
EDIT: While posting, I saw your response. Sounds like you are making great progress!