In my opinion and experience I never got a really good slap tone trough the Ampeg amp, and I tried several preamps.
It's different in terms of speakers, they sound very good for almost every things, try your cab and your jazz bass with a different amp/pre, I'm sure you'll notice big time differences.
Another thing to do is to start with a super neutral tone on the head and see how you can improve it, also if I think that amp and the cab is not the best fit for a modern Slap tone. The 2X12 works on a different frequency field compared to a 10" cab, you can handle a good slap tone with one of the newest 1x12 cabinet on the market now, but 2x12 is far away from a 1x10, 2x10 or 4x10 or even a 1x12.
Another thing, when you use different bass on different rig never compare with your staff, basses are always different from each other; it's different and it's ok when you try your bass on a different rig, but not different bass on different rig, isn't not goo in terms of comparison.
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