Although I don't have the technical know-how to do so, you can fashion a Toslink converter to run the digital signal out of the 8455 to the H701 and get the processing. Although you would need the C701 to control it and you would loose your volume adjustment for the radio on the 8455.
If Squeak is reading this he should be able to fill you in on the details as he helped show me how it exactly it works.
However, as it has been said, by the time you are done buying the 701 combo you are well into the 9255 range.
I think its time for you to design the new ZFactor headunit that has everything all us car audio nuts desire in a 4-way active unit with processing galore!!