Don't let the purists scare you.

Go with it if it sounds better to you. Reproducing what the filmakers intended is an elusive goal for a variety of reasons, the biggest being the different acoustic space you have at home.
Other than balancing playback levels, I say anything goes(even that is sometimes compromised--alot of folks run their subs hot). Yamaha does have some darn good DSP modes, so if you like 'em, use 'em.
I don't really use additional DSPs in the sense that you do, but I do often force 6.1 from 5.1 sources, and I really like it when some older soundtracks are mixed for multi-channel. *GASP* Anathema!
DJ