The Peavey classic 50/50 is a great power amp on a budget, but IMHO - its just that, a budget power amp. I've had a ton of power amps, and it does come down to pairing the RM4/M4 with a great power amp really unlocks this stuff. My personal fav - Fryette/VHT 2/90/2 (which I now have again - and should have never sold) - something about it just makes the pre more 3d sounding. Mine is loaded with Tong Sol 6550's and is incredible.
I've still got 5 power amps - the VHT, three Mesa's (2:90, Recto 2:100, and a 20/20), and a BK Butler MosValve - and the difference between them all is big. I like the MosValve - since its tube less - and as such a great backup - but it just doesn't sound great to me. I'm keeping the 20/20, as my backup rig is a 4u rack with a RM4, Lexicon G2, and the 20/20 for a compact grab and go, or great backup. The recto is cool, sounds big - but not for me. If I were to keep another power amp as a backup/alt rig I'd keep the 2:90, its just a great power amp, along with the modes on it - makes for a great setup.
None of them touch the Fryette/VHT, but thats just me (again IMO). Its the perfect combination of tight, 3d, punch and presence - with the voice switch in - a metal monster, switch out - all kinds of blues to rock to hard rock and beyond - and the cleans are stellar. I think if you changed out your power amp for something like the RT2-50, or a Mesa, or a VHT - especially with the modded mods would probably not even think about the GSP1101 (though I really have to give that processor props - I had one, two Axe-Fx's, and really thought the GSP, configured properly, could hang with the Axe - just not as flexible). I did use both of them for a while with a 4CM setup in my rack - but in the end went back to the Lexicon's.
I also think that the EL84's (and I have a RM20, was using the RM22 for a small show amp, and have the mesa 20/20) just can't hang with the bigger tubes for higher headroom stuff - now if I were looking for more power amp saturation I would probably like them more. For me - when I ran a RM100 I initially used a pair of 6L6's and a pair E34L's - which I think sounded really good - but liked the quad of 6550's better, and ended up with the 2/90/2 to keep that tube.
Another honorable mention, for a killer low cost power amp is the Carvin T100 - not sure about the newer TS100's, but the older T100, with a properly biased set of E34L's, is a great power amp - and it can run as a 50 watt stereo amp, or a 100 watt mono amp. I had one, sold it when I switched back to a RM100 - and would have kept it as a backup over any of the other amps I own now (besides the VHT).