Those preamps are all very different!So for rack mounted/modular pre amps I see: Alembic, Arkham, Demeter and now Broughton
Alembic uses a Fender tone stack and Arkham has certain similarities to B-15's- Demeter: I am unsure
or and it appears that the Broughton shares some DNA with Ampeg
Anything else?
I have been looking for a SVP for a couple of months....
I have an Arkham Zephyr, and I recently tried out a Demeter that a friend was selling. Both are good preamps, but the Demeter is super clean, while the Arkham has more saturated tube character when the gain is turned up. I think Demeter has a more Fender-like tone stack (it seemed to have a bit of tendency to be slightly mid-scooped). I really like the EQ on the Arkham, it's Ampeg-like in that it has James EQ, but its "range" control adjusts the frequency at which the bass control crosses over to the treble control, which makes it pretty flexible. I especially like it for recording.
You might also want to look into Sushi Box FX, a variety of tube preamps and such in pedal format. I just picked up their Underground Accelerator, which claims to be influenced by the Alembic F-2B (which in turn was derived from the Fender Showman preamp). I'm still experimenting with it, but I like it so far, and it's very reasonably priced.