I can't say much about the radio shack receiver that you currently have, but I've had the HK3480 for a year or two now paired with M60's and I've been very impressed. No matter what volume I choose it always sounds great. In fact, I occasionally use it as a 2.0 home theater system and it sounds wonderful there too!

One question though. Why do you feel the need to upgrade your receiver? Do you feel that there's something lacking in your current setup?

M60's
VP150
QS8's
HSU STF-2
Onkyo TX-SR805
Audiobytes for the PC