The funny thing is the older alembic F2B preamp is nothing more then a Fender 2 stage typical guitar amp.
There's plenty of lows in that design you just have to know how to get them.
Make a BFChamp amp with a bigger OT and a 6L6GC power tube w SS rec. Tweak it a bit, maybe adjustable NFB and variable cathode bypass cap/resistors. Then run it through a good quality 10"-12" bass speaker, not one of those low SPL speakers that handle 1000 watts, just a descent one, even the Eminence BP102 might be good. As long as the SPL is high and the XmaX is long. Then take one of those car sub woofer boxes which are simple/cheap and have a shelve for the lows. That would probably make a GREAT amp to use and louder then you think because of the efficiency of the lows from the speaker and cab. Modern 2x10 cabs are not efficient and are a power soak. You'd be better off with an old SUNN 115 cab and a D140 JBL.
Maybe use a beefier PT like the Allen Champ/Princeton one and use a KT88 or 6550 w/ss rectifier.
There's a lot of low wattage guitar amp designs out there that can be modified to become some very interesting low power bass amps. You can add a direct out for recording or a PA and use it live as a monitor too. The thing about them is their TONE, great tone, a mini B15 for instance using a 6SL7 preamp tube and a 7591A power tube. The tone will be superior to anything you have with wattages in the hundreds, even tube amps. Simple tube circuits tweaked properly will deliver great tone and great tone happens at lower wattages. Once you get up there it's raw power over tone, that's the way humans hear.