Who said there will ever be a new expansion board? There very likely won't be, I'd say - Kurzweil must focus on new products now.

If you want to use custom samples, get a PC3K.

Have you tried routing the bass sound to the second set of outputs and running that through a separate channel or amplifier? That's how I roll when I have to play LHB. I don't think there is another open slot in the PC3K8.
There's a lot of stuff in Kore 64, I don't know if there are bass samples there, I haven't made my way through the whole expansion.

When I owned my Ensoniq VFX the best bass guitar was built off of an electric guitar sample that was octaved down. Might want to give that a try.

Probably the samples aren't the only subject here. Often it's what is in the samples and emphasized or extracted out of them that counts towards the "bass sound" experience, so it isn't that the "perfect sample" will suddenly become the ultimate musical instrument. There's enough software that has tried that simplistic approach: V.A.S.T. isn't primarily about that.

thanks for the advises.
1. I always route the bass from the aux output for the sound man in gigs. Still not satisfied...
2. I don't think there's bass samples in the kore 64.
3. I know I could work with the factory samples to create much better bass presets for my needs, just don't have the time right now...

Benny