Preamp tube

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If you want more of that warm sound with good bass  go with Mullard 12AX7 (not cheap)
If  you want clarity and detail with good punch and bass, the Telefunken12AX7 can't be beaten, also not cheap

Another recommendation is to contact Brent Jesse (I'm only a customer), here's link to his webpage for 12ax7s , aside from his listing of what he has available he provides some views on the sound of different brands which you may find helpful regardless of who you buy from
http://www.audiotubes.com/12ax7.htm

I think I may have posted the same question twice on AG, sport about that. I’m getting great responses from both and I appreciate your input. How critical are the 12AT7/ECC81 to change? Is there a sonic benefit? The most I’m hearing are the AX7’s play the most critical part in voicing. WhT do you guys think?

Depends on the layout of the amp, which I'm not familiar with.  It all depends which tube or tubes handle input.  Similar tubes may be just phase inverters, so less crucial.  Mullard, Amperex Bugle Boy, and Telefunkun are all good but on spectrum, with ABB in the middle.

I’m sorry, what is ABB referring to?