Newb Tube question


Having recently acquired a Pro Ject Tube Box S2 phono stage, I'm liking what I hear and am wanting to learn more about tubes.
I see reference to the plates inside the tubes... "short plate", "long plate", "smooth plate", etc.
Is there any sonic difference with the different plate characteristics, or is the quality and manufacturer more important?
Are there brands to seek or avoid?
Just curious... Don't know if my lower-end *system or my 65 year old ears are capable of resolving the difference between tubes anyway.

Thanks!

*System:
Emotiva BasX PT-100 preamp, Emotiva BasX A300 power amp, Zu Libtec speaker cables feeding Maggie 1.6's, Morrow interconnects, Technics sl-1200 mk2 w/ Nagaoka mp-200 cartridge and before-mentioned Project Tube Box S2  phono stage.
mwinkc
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 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.