What equation? I would be surprised if the 7400 wasn't capable of 105W/ch all driven. I did find a review that states Marantz rated it with all channels driven:

http://www.goodgearguide.com.au/review/home_entertainment/marantz/sr-7400/221579#cb

I think to see, well....., hear any increase in SQ you would need to go with the XPA5. The minor increase in output from the 7400 to the UPA would yield minimal if any SQ benefits. The XPA5 should yield 1-2 db louder playback and be all that much cleaner/clearer while doing it.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70