8khz is still quite high. A lot of people like putting keys through guitar amps, and they top out about 5khz?
Depends on the sorts of keys sounds you need, and what your tastes are, I guess.
Might try my daughter’s electric piano (Kawai somethingorother) through my micro cab this afternoon to see what it sounds like.
I love the idea of a 2x6. Got the cab building itch again…