If your Onkyo has preouts, I think you would see a MUCH better boost in performance if you got a new amp and a new subwoofer instead of a new receiver or preamp.

If you want to go with Outlaw, I would get the 770 and 2 LFM-1 subwoofers.

If it doesn't have to be Outlaw, I would get the Crown xls 402 for your mains and let the Onkyo handle the other channels since you mainly listen to music. For the sub I would get an SVS 20-39PC+ or a PB2+ depending on how large your room is.