So you are finding the 125w x 6 is too little?
How many channels do you want to amplify; all 6?
What budget?
What is your planned method of connecting an outboard amp to a typical H/T receiver? I do not see any out jacks on this model?
"Buttery sound" of music or multi-channel audio? Are you just not satisfied with the quality of your sound?
Also, an AMP with only amplify what sound you feed it, not improve the sound? So maybe a new quality H/T receiver or a switch to a top "separates" system.
What is your room size and construction type?