I have a K&K Pure preamp that I use for my upright; on a whim I plugged it in between my passive Bart-equipped fretless 6 and D-180. Very nice- bumping the mids would seriously magnify the mwah; dropping trebl/mids & bumping the bass made it DB-esque. If you have one of these or something similar I urge you to try it on any passive bass- it did nice things for my 73 P as well.
I have retro-fitted a K&K pickup on my Godin A4 fretless and it is miles better than the stock LR Baggs piezo pickup. Just to add to the K&K love-in. Davo