I haven't tried many other poweramps than my RT2/50. But I can say this: the combo of RM4 + RT2/50 sounds like it should!
I ran my RM4 through my Laney combo's poweramp for a while, it sounded good.. but can't hold a candle to the RT2/50!
On the GSP: I'm planning on getting one for FX and odd patches (where I want some extreme preamp setting not worth setting the RM4 up for).
I have an MX200 now and I do not recommend it.
The sounds are fine, but it does not have a killdry function.
So you could set the mix to 100% wet and use patch volumes right? Wrong, there's no volume per patch (only a global input volume).
Using it in the series loop is a no-go as well because there's a really annoying silence when switching patches (like the old RM4's but way worse).
The GSP does not have this problem, it runs on 2 AudioDNA2 dsp chips instead of the MX200's one AudioDNA (version 1).
According to Digitechs marketing that's 8 times as powerful. (apply a pinch of salt)