Hartke LH500 is as simple an amp as I've ever seen one:
Hartke — LH500
Never played it, never heard it though. So other people have to compare it to your GK.
Or LH-1000 if you need to get seriously loud. I like mine, although I've never had to turn it up to seriously loud.
Interestingly, my "backup and acoustic/coffeeshop" is a GK MB500Fusion.
I run Presence and Contour off and all the EQ and Gain at noon. Then Master is just Volume, and I'm done.
I've only ever used one "channel" although I've played with bumping up the gain (and down on the Master) on 'B' just as a test.
It could also be handy when I'm doubling with the EUB, with the same Gain but higher Master, 'cause I think it's a little quieter out of the pre.
So for me, practically, the GK and the Hartke are 'the same' - I never move the EQ and Volume is just Volume.
There's plenty of EQ in the Roscoes' 3-band Bartolini pre's, and I have pedals if I need drive or more EQ.
Can't help you with cab choice, tho. Lots of suggestions up there for cabs that get a lot of love.
I have 2 - a GK 4x12 Neo for "big & loud" and a Hartke 1x12 Neo for practice & acoustic support.
As the OP shows GK sponsorship, I'm thinking there must be a GK solution...