Modeling a preamp after the Bassman is nothing to shy away from. The Alembic, Kern and great guitar amps like plexi Marshalls and Soldano have their roots with the Fender Bassman. MESA/Boogie got it's start by hot-rodding Fenders.
While Bass Player magazine uses a Demeter-Hafler rack as their reference, BP's review of the Kern blew the reference Demeter rig away! However, that is their opinion and ears; not yours. I got so burnt out demoing heads and cabs. I tried and fell in love with the Kern and called it quits.
I did not try out the Alembic and Demeter preamps. The older Demeter is supposed to be very nice but lacking in low end and with relatively functionless tone controls. This isn't as major a setback as it might seem if you have a very nice bass with superlative on-board active eq. The new Demeter H has a parametric eq which might have remedied the situation. I've heard people describe the Kern as being similar to the Alembic but with a bit more growl. The Kern cab be orderd w/o the wall wart power supply. Nevertheless, despite all the hyperbole above, use your own ears to judge.
If you've got some extra bucks and don't have local access to these products, maybe call a place like BassNW. Ask them to ship you the Kern, Demeter H, and Alembic preamps. See if they can work out a deal like splitting the shipping 50:50 or 30:60 if you decide to keep one of the preamps. I think they have a 72 hour approval policy. Kern will ship you a preamp direct for a longer trial period but my charge more than a retail store.
Good luck.