Preamp for Kr Audio Kronzilla SXI

I have a Kr Audio Kronzilla SXI integrated amplifier, which uses a passive preamp stage that is basically a volume control and switches for inputs. While I've been very happy with this amp, I'm wondering what kind of improvement I might get if I added a nice tube preamp to my system and bypass the passive stage.

I'm interested in preamps up to $3000 new or used. Some that I was curious about are the Dodd battery-powered preamp, DeHavilland Ultraverve, CAT SL-1 Mk3, Lamm LL2,Lector Zoe, but really just getting my feet wet.

Any advice would be appreciated. thanks.