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mark bass or gk backline are some super compact bass practice amps. however, you don't want that for a drone synth. what you want is a full range speaker. either a roland keyboard amp. or a qsc K8 or K10. I would not recommend the behringer keyboard amp. JBL powered monitors are ok if you get one used for a good price no issues. out of all those options I would still lean more towards a K8.

Any bass amp should work HOWEVER...

I’d make sure you can put your guitar amp on top of the bass, which creates a vertical array of speakers. Otherwise your sound will feel very divided and will not sound good. I’m doing this with backing tracks right now. When i had backing tracks stage L and guitar stage R I couldn’t get the levels to feel balanced nor get the sounds to sound like a band. It always sounded like “karaoke” where it was X and Y, not X WITH Y

In my experience, Roland keyboard amps are full range, but they have a punchy voicing and they add some grit when you turn them up. Works well for some keyboard sounds (rhodes, organ), but they aren't a good substitute for a PA speaker with a more neutral sound. It just depends on the sound you want