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 .

Rrekstad ;
This is from Onkyo's website :

TX-SR605
7.1-Channel Home Theater Receiver

HDMI V1.3 repeater (2in/1out, 1080p compatible)
HDMI video up-conversion with DCDi technology
Dolby Digital Plus, Dolby TrueHD and DTS-HD decoding
90 w/ch x 7 ch

What audio formats are you refering to ?