I was looking into the same thing, this is the advice given to me.
Posted by thehardknoxlife (from the My Pg Rsd65cs Review thread)
The older Momo series MM6 are similar in ways. Alot more than the new series. IMO the MM6s are superior to the new set(take more power,get louder,extend lower etc...). Even so, the RSDs are better performers than the momos. The only aspect the momos might have on the RSDs are in the lower mid-range even then the price difference doesn't make up for it (especially if you have eq to tame the rsds). It's really a no-brainer when considering the two $175 set v/s $90 set that performs just as good if not better(better by a long shot IMO espcially if your considering the MMC6500).
I am still undecided, but i will think I will try our the rsd65cs' when i get the time.
Hope this helps.