Newb Tube question

I am about your age and a relatively recent tube-a-holic!  I think I love them because some are older than I or at least the correct generation.  I found the following write up / review to be very helpful although I do not agree in any way with his blanket indictment of 12AX7 tubes...having run them all I have some favorites.  I also found Brent Jesse's website to be a great wealth of information.  Have fun and enjoy the journey!

https://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html

OP;  I concur with the above comment about critical listening. During a listening session, even if you have high frequency hearing loss, you can discern the differences between tubes. Some are very obvious such as Telefunken (wide open transparent image, very dynamic) vs. Mullard grey plates (warm with a rolled off top-end). 

I love tube rolling and now own an all tube system; amp, pre, and dac. I can hear the difference between each tube swap so I'm sure you'll be able to hear the difference in a single component. 
My recommendation is to try NOS tubes which have extended highs and cast a wide soundstage. Get as much high-end information as possible from the tube since your hearing is causing a high frequency roll off.

Sylvania tubes have high-end energy, good detail and a transparency that is very revealing. RCA blackplates have lush sonics with a wide,  deep soundstage.
IME, you will not hear drastic differences between the new production tubes made in Russia. They don't have the different flavours as NOS do.
 
 

Thanks to all for a compendium of great advice and resources.
I'm very happy with the way my tube-phono stage and upgraded stylus sounds..to the extent that I'm going through my vinyl collection, enjoying the adventure of hearing new, better details and color in acoustic instruments and voices. When this blissful stage has run it's course, trying a different set of tubes or two will be a fun way to tweak the sound without breaking the bank.
Without your great suggestions I wouldn't have had a clue where to start.

Sincerely,
Mark

12at7 and 12ax7 are compatible? 

12at7 and 12ax7 are not common subs, but some circuits do allow that in certain positions. The Herron VTPH-2A is one such example - using 4 of 12ax7 and a single 12at7 yields another 5 -6dB or so of gain over 2 of 12ax7 and 3 of 12at7. The 2 slots allowing the sub may be mu followers? 12ax7 are not usually used in follower positions, though I guess some phono stages do that. In that case a 12at7 should be a fine sub.

Definitely don't swap them unless you know it's an OK choice for that slot.