My main bass amp is a Carvin BX500. Carvin stopped producing amps in 2017, but up to then was pretty well known in the US. I gather that it's not that well known outside the US... Mine has class D power, with lots of tone shaping knobs (drive, parametric eq, graphic eq, treble, bass and contour and compression), and weighs in under 6lbs.

It's mostly solid state, but has one 12AX7 tube which can be engaged or disengaged. I thought this might make the sound a little warmer, or change the sound of the overdrive. What it seems to do, however is nothin at all. I am thinking of swapping out the 12AX7 for some other tube, but I don't know much about this and wondered if anyone here had advice about that...and what I might expect by changing the tube?