moretolife, I have two fretless B2's, a 4 string and a 5 string. Both of mine have the humbuckers on them as I only use the neck placement pick-up so the single coils would make noise. I love the single coil fretless sound, but I would rather have a quieter bass then a bass that buzzes and makes me stand in weird positions to get rid of the single coil buzz. Since I would only be using one pick-up it would always be a challenge.
I also have the hard as nails finish on the fingerboards as that helps the growl and saves the wood, as I use round wound strings on my fretless basses.
Both of my fretless basses have the piezo pick-up option as they really shine on a fretless. You can get almost an acoustic bass sound with them using them alone, and if you blend them with the mags just right it increases the bottom, thump, and the sustain.
Other then the hard as nails option my basses are bone stock. Carvin has so many wood, hardware, and finish options it's truly like being a kid in a candy store, so have fun trying to decide!!
Those are just a few thoughts as to what I use, but it's all about what you want and what you feel would be best suited for your needs. I love my fretless B2's a lot. If you want to hear them I used them quite a bit on my 'Bromberg Plays Hendrix' CD. Not trying to do a sales pitch but you can really hear the 4 and 5 string versions of the fretless basses.
Thanks for asking and good luck with your bass. Please let me know what you decide and if you choose to get one please take pics so I can post them on the Carvin B2 Bass Facebook page. thanks...........B2