under $500 used which a/v receiver or processor

I think that the pre/pro route may be over your budget , for a decent one anyway . You may have better luck in the A/V receiver market .
You should be able to get a nice 5.1 , without HDMI , A/V receiver in the used market for $500 . If you want HDMI , Onkyo's entry level unit (605 ?) can be had within your budget , online new .

Good luck .

I second the Onkyo 605 for $399 at Amazon. In addition to HDMI connections it decodes the advanced audio formats (Dolby True HD and DTS-HD Master Audio). If you have, or plan on having a HD display, blu-ray, etc. in the future, you'll regret not having these features.

in terms of a more detailed and accurate presentation; which brand do you prefer b&k, nad or marantz?

wait a little longer and save up for the Integra. its the best thing out there at that price level. For $850.00 brand new you can get a 6.8. It's got 3 HDMI in/out, 1080p, 7.1, has 3 zone capability, its THX certified, has Dolby True HD, and independent block construction. Also is XM/Sirius capable.