I have the Sherwood Newcastle R771 (C$600, much cheaper down south) and really like it (also recommended by Axiom for driving 4 Ohm loads). If you need HDMI the R772 I believe is MSRP US$600.
As for Yamaha the RXV1700 at 130watts is in the $600 range.
where had you seen the 1700 for $600??
They were on sale up here before and just after Christmas for $600. I am willing to bet some of the online retailers have them for this price.
Unless you want some of the newer features available on the newer avr's, you really don't need to worry about your present one. It has plenty of power for the M60s.
