Another vote for upgrading the speakers first. Your current receiver will drive them just fine, and the improvement from upgrading the speakers will be MUCH greater than the improvement from upgrading the receiver.

Just don't get M80s -- the Sony receiver might have trouble driving them because they offer a 4 ohm load.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8