Oops. I once played a Hartke with my double bass. Not the DB sound, but you might use it for a while with a different pickup.
You need to think about how much money you would like to spend over time. Getting a good but expensive pickup for a "bad" amp now and maybe get a different amp/Cab for DB (and EBG?) later or getting a better but not too expensive pickup for your double bass.
The cheaper solution (where you don't need a impedance buffer because it is built in) is the Shadow SH-965NFX. Only few people use them in the US, but those who heard them think it sounds at least OK. It is one of my two mostly used pickups (the other is a Fishman Full Circle that replaces adjusters, needs a preamp and probably would sound less good with your Hartke equipment). If you think it sound a bit echo-ic or flanged, try to connect the bass side foil only (this is possible on either jack). You need a battery and should take a spare one with you. Don't let the plug connected to the pickup if not in use or the battery is discharged after a few days. And set the tone control to high, it is not useable. It is also relatively easy to mount (after putting the string tension a bit down to be able to lift the bridge foot).
The more expensive variant is the Ehrlund Pickup with it's preamp. At least twice the price (I think even more) and you have to search a while for the sweet spot where it sounds good.
Both pickups are good for pizz and bowed use.
There are more pickups available and others may direct you to them, personally I think the Shadow NFX is a "better" (a bit brighter) version of the Realist. The bridge wing pickups are less good for bowed use, the BP-100 the worst for the bow. The Wilson lacks the body resonance and is expensive. Magnetic pickups sound ... magnetic and (a bit depending on the manufacturer) not like a double bass and need steel core strings.
For DB slap-style, you need a second pickup at the end of the fingerboard to amplify the slap.
My personal recommendation:
Get a set of better strings and the Shadow NFX and try to dial the Hartke for a uncoloured frequency response. And maybe remove a bit of lows and highs, a DB cannot reproduce low frequencies well by it's body and the same sound should be produced by the amp and the hartke cabs/amps produce a lot of highs (for that EGB slap thing) that makes the DB sound too hard sounding. Low (and maybe a bit of high) mids is what the DB likes.
I hope this might help you a bit.
