Alembic F-2b "red knob"

[Rhythmdoctor]

I'm new to Alembic - got my first Alembic about 6 months ago, a 1983 Spoiler - and I'm ADDICTED to the wonderful tone. Now I'm in the process of upgrading my amplification as I'm currently running a GK 500watt 2x12 combo amp.  Anyways, I'm shopping the Alembic preamps and just noticed a listing with "red knob" and I don't know what this means. The images on the Alembic website don't show red knobs.  Also, while I'm on the topic what is the main difference between the F-1x and the F-2b?  For bass amplification what is the difference in the two?  TIA.

[lbpesq]

“Red Knob” means it is an older unit.  They originally had red plastic inserts on the knobs.  My F-2B is a red knob version.  Works great and is otherwise the same as newer units.

Bill, tgo