Everything being equal, the 5X100W amp should have a larger power supply assuming the 985 has just 1 large power supply. The problem is, if you biamp the Studio 40 with the 5X100W amp the bass driver will only get 100W. 100W Rotel power per channel may sound like a lot but in general you don't approach the point of diminishing return until you get above 200WPC.
For this reason alone, I would think that you are better off with the 2X200W amp because you will end up with more power for the drivers that needs it most. Another point to consider is that biamping is not much better than biwiring unless you also have active crossover systems and disable the speaker's internal passive crossover network. Also keep in mind that all the perceived benefits of biamping are theorectical, there is no guarantee that such theorectical benefits would in fact translate into improved sound quality that is audible to you. More power, on the other hand, will at least minimize the possibility and the severity of clipping that is in many cases audible to most people.
By the way, thanks to you I searched and found the RB985 manual. This is the only amp so far that actually specify what 40 amp peaks mean. It is for 0.1 ohm load, 10 microseconds, 1 pulse. So power=40X40X0.1=160W assuming that 0.1 ohm is purely resistive (if not, it will be even less), and this is Rotel.. So much for the high current misconception! Still the spec of this amp is quite impressive but it only weighs 34 lbs.
