... I don't know how to describe it (as somewhat of a noob). I play thru either my Avatar B210 (non neo) which I like OK, or my guitarist's GK 210RBH which gives me the sound I hear in my head. I don't know how to describe the difference other than to say I think I hear more fundamental in the GK. Can anyone help me to describe what I'm hearing? Also I can't find an RBH locally, are there any other cabinets (prefer 210s) that present a very similar tone? Apologies in advance - I know I'm asking a poorly defined question.
RBH are the top line G-K ceramic speaker cabs. Unfortunately, the 210 is no longer in production. Only the 410 and 115 these days. I can't help with a similar sounding cab, sorry.
I found "the sound I hear in my head" when I let a Low B roar through my first Acme B2 cabinet. It made me buy three more. I would talk to your guitar player and see if he is will ing to sell you the GK. Then start the search for a second. If he won't sell you the cab look out for two of the GK cabinets. Even though they are out of production some should show up on the used market. An alternative is to try out some of GK's more modern offerings. They may well have on that sounds close to what you are looking for.
The new CX series might come close as they are "traditional ceramics". They just started shipping so not many reports yet.