ZiggyZoggyOiOiOi, there are a great many people out there doing exactly what you are thinking about doing .

Not all but most rec. have respectible processing power built into them . H&K utilizes Cirrus Logic (crystal) chips albeit different than the ones incorporated into the 990 . That still says alot about their build quality .

I myself was using a Yamaha RXV2500 as a pre/pro in my system until my brother talked me into loaning it to him . When I first started to use the Yamaha as a pre/pro . I noticed a substantial difference in sound quality right away with a more robust fuller sound that had a wider range of dynamics to it .

I can only think it will improve and not hinder your sound quality by utilizing seperate amps and bypassing the internal amps of the rec. .
It will take alot of the grunt work off the rec. and allow it to preform better .