feedback at around 1.5 kHz
 

I've been experiencing high pitched feedback (around 1.5 kHz) in live situations. I am playing an alleva coppolo lg5 bass, always in passive mode. The amp is mesa boogie mpulse 600 into aguilar db410 cab, horn always off. I keep bass and treble on the amp most of the time flat with occasional slight boost of treble, middle around 2:00, 3:00. Gain is usually set between 12:00 and 3:00. Feedback occurs without any pattern for couple of seconds and disappears on its own. It happens more often if I tame the pot for bridge pickup or completely turn it off, but also when both pickups are set to same volume. The feedback occurs only at concerts and not at rehearsals where I use different amp (ampeg svt 3 pro) and cab. I don't know what is the source of this problem, the single coils of my bass or the amp or maybe the combination of both? Any help would be really appreciated.

Sounds a lot like your pickups being microphonic. It's when the windings of the wire inside physically move and feedback. Generally you gotta either get your pickups potted with wax, or replace them with ones that are.

Thanks for the tip, I will check about that.