Hey gb, if you do decide to sell your MP2 I'm interested and it would go to a good home
Trouble shoot.. those symptoms sound like connector problems (jack sockets, leads), if you haven't changed your guitar jack socket out in a while it's allways a good idea, they take lots of ware and tear. Don't skimp, put in the best quality you can, this is the first fundamental connection in the chain and they do wear out..
Confusing, no problem with MP1 but is with sp77 ?? could be that the connection issue is intermittent ? or issues with sp77 (possibly input jack needs a clean with contact cleaner, can't hurt BTW and basic maintenance). So that's the common stuff.. Could be an issue in the sp77 as crackling is random (could be just the fridge turning on off in the power circuit (solution power conditioner or uncoupled UPS)). Check leads power board(s) etc
I've got 3 power amp vol setting opinions/experience.
In a PA setting (as I'm an audio eng), you'd normally turn all power amps up full, gives best headroom and control overall vol from desk (good gain structure)
In PA gear where you want to over power the speakers (i.e. you have more power amp available than speakers can handle e.g 500 w amp > 300w speaker), valid scenario, even better head room, turn amp to 3/4 ish, carefull can blow speakers..
For guitar rack setups, I tend to go between 1/2 to 3/4 on power amp and around 1/2 or a bit more on MP1/2. From a gain structure context, I wouldn't go less than 1/2 on power amp, and adjust MP1/2 input to suite overall vol. However, if you've got other stuff chained between MP1/2 > power amp, it will change everything as now you have to get the gain and the wet/dry mix right through each of them. Anyway give me a call anytime we can chat at length