+1 try setting speakers to small crossed over to sub at 80hz
Strange you should be having this problem. The Onkyo is supposed to be a 100W THX Select 2 certified unit, so if those ratings are accurate and/or meaningful then you should have no problem driving M80's + VP180.
Also, you should not have been "blown away" by the differences between the Pio SC27 and Denon 4310. Both are excellent units and I'd be willing to bet that any significant differences you heard had more to do with (intentional or unintentional) setup/setting differences.
