but couldn't get the speakers to stop "farting out".
I like the way the GK sounds on its own, but its tone is rather dull.
You did not say if you bought the GK second hand, or brand new. That "farting out" speaker sound is by default "speaker distortion" and different from amp or preamp distortion. And is very less desirable when it comes to bass. Since it comes at 200W (although switched) I find this very very peculiar. There was a MB 115 mark II released somewhere down the line, I don't know the rationale behind that one. What was bettered with the II version compared to the first one. Or just more features.
I'll suggest - as before - if you have the time and possibilities try your bass with another GK 115 amp, preferably brand new at a store to see if it's farting out. You said earlier that it was the lowest notes that it started to fart out. A 15" at 200W should be taxed pretty loud before that even happens. This is with the eq settings flat of course. You told us you have some other amp too? Peavey head ? How much wattage? Same thing there?
There might be some problems (i e fault) with your GK after all, since that farting out is entirely different, from all kinds of preamp distortion, even if it's from a pedal. I don't think in terms of blown speaker, but some kind of other thing, with the power amp, some crossover, or a component inside that may have gone south.
You said originally in the OP post: "70's Peavey Centurion head with an older 1x15 cabinet." Have you tried your GK in that cabinet, does it sound the same? I don't know if you can use the GK and hook up another extension cabinet to that one. You will move a bit more air in the low end, but the overall volume may sound softer and not as loud. It depends on the OHM settings. I don't know.
All this seems a tad elusive to me. Mind you that GK manual says (among other things):
*Some passive instruments can have output
levels similar to active electronics. If the sound
is distorted the -10db pad should be used.
As I think you've been through it all by now, in a sort of "tried that, been there, done that" trial and error procedure, it starts to lean towards "something else". IMHO.
- - - - - - - -
Regarding the Fender Rumble
panegyric around here:
I am on my third combo now Fender Rumble 200 W on warranty repair. 2 of them crapped out on me (distortion, farting out, low levels all of a sudden, once in the middle of a tune at a gig) before warranty period, and I had to change them out. Of course no problem with the music shops. Not taxing it anywhere near levels of havoc. Not anything above 12 a clock, and passive pickups. They do sound great though ... when they work.

I'd wish I'd stayed with GK...ha! If this one happen to crap out on me again, it's definitely over-rumble for me!