I just bought the above tubes to try in my ZP3 and Jolida CD player.  Currently I have Raytheon 12AX7's in both...bought from Mike here, which are very nice.  I just wanted to try something different.  Looking at the codes, I think these are from 1955.  I'm not sure why but I was the only bidder so I got them for $125 plus $8.90 S&H. I've given the link to the auction on ebay.  Take a look at the pictures and listing and tell me if I'm missing something...these should have gone for much more in my opinion. They are highly rated by Joe of "Joe's Tube Lore".  Has anyone else tried these in the ZP3?  Mark.

The auction write up..."Four 5751 / 12AX7 GE Black Plate tubes.   As seen in the pictures, matching date codes are on each set, meaning that the first two match, and the second two match.  These tubes read extremely strong.  The readings are as follows: 1725/1725, 1700/1700, 1650/1650, and, 1700/1725, and are therefore, very well balanced."

What Joe said..."GE 5751-WA Black Plate - This is a '50s production tube. Many tube freaks say its simply a relabeled RCA, but I own RCAs and there are subtle internal differences and rather dramatic sonic differences compared to that brand. This tube is often found as a military boxed government contract tube (ex.:JG-5751) and is always in my experience boxed in a white date coded box. They invariably have production dates in the 1956-1959, or so, range. Its got shiny black plates which are shaped differently from the later GE grey plate versions of this tube. OK, enough physical description, how does it sound? Well to my ears its a more refined and yet more lively tube than those in group one. It has a good linear frequency response with fairly good HF extension, if not quite as palpable in the mids as a few others in group 2, but not by much. It measures like a dream too. Like all early 5751s I've ever tested (1955-1965, or so) these guys are unbelievable tight in terms of gain, low noise levels and vanishingly low microphonics."

http://www.ebay.com/itm/Four-GE-5751-12AX7-Black-Plate-Tubes-/191106091394?_trks...

Unfortunately a fair comparison will have to wait. Before I put them in the ZP3 my Torri MK IV Pooped out and had to be sent back to Steve.  I did listen to some vinyl with a pair of Senheiser HD 800 headphones into the CSP3 just to make sure the tubes worked  but these headphones have always been too bright for me.  I may move my Taboo MK IV back into the Listening Cave but I didn't like the way my Zu Audio "Soul" speakers performed with that Amp...I guess I could do an A/B with my Audeze LCD 2.2 Cans. I'll eventually post my impressions for you but I wouldn't hold my breath if I were you...It may be a long time.  Then everything might sound different once my HR-1's currently being built by Bob, are in the mix. The same ebay seller had four more for sale with the same  $125 starting bid. I just looked and their are no bids and the auction ends in an hour and a half.  Mark.

PS...I could put a pair in the Jolida 100 I'm using in the den with headphones.

http://www.ebay.com/itm/191118161310?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m142...