12AU7 and 12AX7 long black plates vs short black plates. Any difference ?

I am almost certain that best Sylvania and RCA  are from 1950s, and the earlier the better. At least, that's what I read, I only have 1950s of both. Don't buy RCA from late 1950s, that's, later than 1956, they are a little dark and closed in; early/mid 1950s 12AU7 RCA sound excellent in my VAC, almost as good as Sylvania long black plates from 1950s.

After reading Joe’s Tube Lore years ago, I bought a trio of Sylvania 5751 for my then EAR 834. I thought they were bright sounding.

5751 are military with lower gain, not exactly 12AX7. 

@inna 

The 5751 has about 30% less gain than a 12ax7.  But don’t think it in any way translates to less versus more musicality.  I know they were also produced for the commercial versus military market.

Based on my use, I agree with Joe of Joe’s Tube Lore as they sound better than 12ax7 when used in the line stage of my preamp.  I have on hand and have enjoyed a number of brands of NOS 12ax7s but prefer the 5751s.

You may have an eye (or ear) opening experience and at a minimum gain insight by trying a good set of Sylvania black plate triple mica or GE black plate triple mica 5751s in your integrated.  These have been my favorite brands of the 5751s I have tried.  

Edit:  Or better yet maybe someone who has experience using 5751s in a VAC Avatar SE can chime in with their experience.

jetter, this could be interesting, if I can find them matched. I did try 12AX7 GE military short black plates and they sounded quite good but were no match for the Mullards that I have there. But right now I am saving funds for another quad of Mullard XF2 power tubes, two matched pairs I would say, matched quad is impossible to find or very expensive.