FWIW, I was in a similar situation - 5 string fretless w/ Bart NTBT - and went on a similar quest for tone nirvana. I went with the ACG filter pre and have been
very happy.

My alternate top choices included the Bart NTMB. I also found I could get the Bart NTMB in a 4 knob form - my bass is drilled for 4.
Much of my research indicated managing the mids is the secret to a great fretless sound.
Good luck with your quest.
