I have used my double bass live for Jazz, bluegrass, roots/folk, and blues. Here is what I have done.
For reference, my pickup arrangement is much more minimal that yours, but I find it works well. I use a Gage realist into a Sansamp ParaDI or even at times go direct to a Countryman Type 85, if I am working with one of those engineers who are afraid of pre-amps and insist I only monitor through house monitors.
I'd forget combos. Been there an awful lot. Heavier, bigger package, inflexible configuration, not upgradable except with an extension speaker, etc., etc. I just don't like 'em any more, since I have gone modular.
I generally prefer a good used 210, like an Epifani t-210 or UL1-210 (~$450+), but have been happy with a modest used goliath III, Jr. ($250), and also liked the Aguilar GS112NT ($used??--don't know) I borrowed for a while. Heck, I even was OK with an SWR 1x15 Son of Bertha.
Just don't skimp on quality in the speaker cabinet. BTW, a tweeter is a good thing for upright, especially if you slap (the real kind) as long as you can dial it in or out as you need.
I think a good used 2x10 or 1x12 paired with a used ~350+ watt (into 4 ohms) head with an
effects receive are what you want. You can by-pass the built-in preamp.
If you could swing a Genz Benz Shuttle 6.0 used ($400+), which is what I am currently using paired with a light 210, you would have a killer rig, but a modest SWR 350 (chrome face) should go for about $300, weights ~17 lbs in 2 rack spaces, and has the right features for upright. In the past, I've used mine a lot for upright and electric, often on the same gig.
In sum, I think you can get away with as low as $550 and might be happier with what you get for $800--all with good quality used stuff mind you--not toys. You would also certainly have a rig that is modular, flexible, and expandable.
As I am sure you know, the EQ section on most bass guitar heads is probably not going to work well set "flat" for you with the K&K pre, and that you are likely better off just skipping right to using your pre as an input to the power section of a bass amp via the effects receive.
For your Ubass, a bass guitar amp's EQ will be more friendly, so that should be straight forward, IMHO.
