Do tubes need break in / burn in?

I've bought a set of NOS JAN Philips 5751 tubes to replace JJ 12AX7 in my preamp. After the change, the sound has become worse. The soundstage depth is flat, and just less involving overall. Do the tubes need break in, and if so how long do I need to burn them in?
I have found about 40-50 hours typical are required for break in of tubes. Some brands and types more, some less. It could be that the new tubes you put in are just not a good match for the preamp. I've had a couple of different pieces of gear that took like 8-10 tries to find the right tube. It's a crap shoot for sure on some gear.
This has been discussed here before. Type in nos tubes in the "search archives" section of this forum and you will see a posted question from 2008 about this same subject.
HI, had same preamp years ago,VTL5.5 and the only tubes that work best was siemens 12ax7 don't even try telefunken they sound terrible in that preamp, another suggestion is that,
VTL 5.5 is not a good match with SS, amps reason why sold my

regards: ,
12AX7 has a gain factor of 100, while a 5751 has a gain factor of 70, which may be the reason that your preamp sounds so poorly. Try using some NOS 12AX7's instead of the 5751's.
In general burn in will take the edges off. However the sound after about 10 to 20 hours is going to represent the end product to a great extent.
Your pre may not have a circuit they will work in as mentioned above. A CD player I bought early in the hobby just made a GAWD awful shriek of painful protest when I subbed them.
I recognize that many will argue that a tube fleshes out with time and it's true, but that takes place gradually over a long period. In the interim, however, the tube will show what it is going to be.
To me immediate burn in has almost always been a smothing up with some getting significantly fuller tones. However Flat earth to halographic soundstage is not what I have experienced.
It really sounds like a really bad pair of 5751s.
When they do work out, they function beautifully as a stage expanders with halographic imagery. I started with Jans and they sent me sliding down the slippery collecting slope.
Some equipment just do not respond well to 5751 tubes..It may well only be that case...Try a pair of Amperex Bugle Boys or Phillips Miniwatt 12ax7 tubes and you should be very happy.I have tried a few 5751 myself and always come back 12ax7's...Amperex,Phillips Miniwatt or Tungsram...
I agree with Cyclonicman.It's a different tube.Conrad Johnson
uses 5751's in some of their amp's,but they're designed for
for it.If your amp calls for a 5751,its a different story.Your
VTL 5.5 calls for 12ax7's I think.It should deserve some good
NOS 12ax7's though.I've had noisy JJ's that didn't get along
with some preamps too.Someone suggested RCA's.Bugle Boy's
sound nice at times too.
I tried 5751's in my Rogue Audio Tempest and did not care for them.I used some NOS Sylvania and RCA 12ax7's with real good success.I then tried a set of Amperex 7025.They sound great and have taken up permanent residence in the Tempest.The music sounds more "real" to me,and isn't that what it's all about.They may or may not work for you.Give them a try,you might be pleasantly surprised.