As an authorized Sony of Canada retailer, I can relate to that experience.
Sony Receivers, unless your buying the top of the line ES series stuff will tend to be pretty gutless amps and it's due to cheap power supplies with virtually no headroom at all to go above rated specs.
I own a Sony STR-DA777ES receiver which is a $3500.00 piece and it kicks ass with authority and ease but their regular stuff is built to fit a price point only.
I'm sure a discrete output Sanyo of an older vintage like yours would sound better but, I doubt it would compete with my Adcom GFA555, 200 wpc power amp which peaks at 325 watts into 8 ohms and 400 watts into 4 ohm. It is a true power house that sounds incredible!
Cheers!