12AX7 vs. 5751

I'm getting my first tube pre (Counterpoint 7.1) this week or early next week, and of course I have start fiddling with it ;-) I've been reading Joe's Tube Lore (http://www.audioasylum.com/audio/faq/joes-tubes.html) and the 5751 caught my eye, however I see statements like "lower gain that the 12AX7" and lower "mu" (what is mu?). Anyways could I use the 5751 tubes in my Counterpoint without causing any damage? altavistaaudio.com mentions nothing about the 5751.

I use a matched set of 5751 NOS RCA from 1959 in my Belles 21A preamp. I went to them from Mullard 12AX7. I found no audible difference in gain with my system. I found the RCA's to provide a little more air and detail between instruments, a little more depth of soundstage and very nice extention and tightening of bass. I got these from Vintage Tubes and have been very happy with the upgrade.
5751s will work fine on a 12ax7 line stage.
OTOH, on a phono stage, you need all of the 12ax7 gain.
Good luck
Typically you can use 5751s where 12ax7s are called for. The 5751s having less gain just gives your volume control a little more flexibility, ie you can turn your volume control without a little adjustment giving you a big volume increase as 12ax7s give in some applications. With my preamp there is a noticable difference in gain between these two tubes. 5751s were speced for the military and are very rugged. They also sound very nice, IMHO. I too like their air and detail and I would add bass response. Just to make sure, send altavista an email, they are the experts on Counterpoint equipment.
There are no guarentees it will sound better with a 5751 set ...Each peice of equipment is a little different,so the only way to know is to try..I tried and like the RCA 5751 black plate ( circa 1962 ). Not much gain loss and gorgeous mids with good bass..Didn't like the Mullard ( to dark for my set up ) .I did however think the Tungsram 12ax7 ( Hungary ) is one of the best 12ax7 nos tubes I have ever tried...( available at Tubemonger.com )To me if I were going to try rolling I would suggest try either one of those tubes mentioned. Both great but slightly different..And as mentioned by Davt ,the Mullards work great for some also..Its a crapshoot!
I have used both A LOT and never noticed the gain difference in 5751s. But they sure sound different. The right tubes will depend on your tastes and equipment.

I have an additional question.

Can a 12ax7 be used in place of a 5751?

the preamp is speced for a 5751 in the line stage.

in many cases you may substitute a 12ay7 for a 12ax7, with a gain difference. i have done this with a cj premier 3, audio note cd2, cary slp 94, mcintosh c220 and nobis proteus preamp--phono stage .
For Fisher 500C receiver the 5751s do not have the dynamics of 12AX7s.