Seeking a Warm 12AX7 Tube for Preamp

GE 5751 Triple Mica Black Plates did it for me. 
I know many folks despise EBay for tubes, but you probably can pick up a matched NOS pair for a few bucks under your budget to try out and see if you like. Make sure to check the seller’s feedback and that the seller accepts returns and you should be fine.  GE 5751 Triple Mica Black Plates are very desired excellent sounding tubes and in most tube reviewers top 10 lists.

@skiznfliz, @trivema  - triple mica black plate GE 5751 are the bomb for sure. I bought a set last summer from a local Mass dealer. Use them in my lineatage and they are just sublime. A better fit with my heavily re-worked Valencias. Super quiet too, what a great tube. Might even replace my Tele smooth plates as my new favorite. Just my experience 

@fertguy OP said warm, which excludes TF. 

@spacecadet65 to find out if 5751 is substitute for 12AX7 in your amp/preamp, contact builder/manufacturer directly.

5751 ands 12AT7 are not interchangeable with 12AX7, despite folks treating them as such. any and all substitutions are circuit-dependent.

if you have a high gain preamp, 10-12dB attenuators are the most direct response. ive youse hlabs 12dB inline RCA attenuators for this very purpose. in my system theyre transparent. 

Grant Fidelity are pricey but high quality.