Well, just my opinion:
1. _tom_s Gh50L clips he made in the past all rule! That amp is great, and I played a GH100L recently and it's amazing!
2. SM57s are great. I have a SM57 and a Sennheiser E609 and a condenser mic, and the 57 is the best. Just very touchy about placement, but it rules for overdriven/distorted guitar
I would keep the GH50L, it's a great amp, and with no band will the HT5 have enough horsepower. Maybe the tubes are done or it needs a rebias. I think you've had those power tubes for a long time now, right?