The amps (350L, 350B, SS rectified 2000S) are very similar. Take it to a competent tech along with the schematic for a 2000S. Tell them that you want the mid switch on your 350L to do what the bass boost does on the 2000S schematic.
That should be a piece of cake for a good tech, in fact they should be able to by pass the "frills" and completely remake a 350L into a 350B pretty easily.
I will also add that a 350L should be a pretty killer bass amp without any mods at all.
The bass boost on a 2000S can be a little strong. I suppose it has a lot to do with what cab(s) you are using, but with a JBL loaded 2000S cab(s) it can be sort of "over the top" bass wise.
IMO the bass boost sounds pretty killer at low SPL levels, but personally I like
my Sunn's "cookin'"; I enjoy flapping my pant legs and tingling my hangy downs as much as the next player, at volume a 2000S rig moves
plenty of air without bass boost engaged, and in my experience using the bass boost can really suck headroom and make things get a bit "inarticulate" and pretty hairy.
Tonally sloppier than
I like to play anyway, as always
your results may vary.