Most of the 12AX7s in current production do not meet the old RCA standards for output and transconductance. They also tend to have more white noise than do the 12AX7s from the reputable tube companies of yesteryear, e.g.: Philips, GE, Sylvania, RCA, Mullard, Bugle Boy, etc. Some new production 12AX7s sound very good, but many will not deliver the quality tone you will get out of 12AX7 made in the golden era of tube production.
5751s were built for the military. By design they had gain factor of 70. The 12AX7 had 100. Spec. transconductance values were also a little lower - I believe 1200 instead of 1600. That said, I have a whole batch of JAN (Joint Army Navy) Philips 5751s from the 1980s that test out in 12AX7 range in all values. 1980s production from GE and Philips are still available, but they are becoming increasingly scarce. The current production 5751s are far inferior to the old 5751s.
The RCA "Command Series" 5751 goes for a premium. Also much sought out is the GE "5 Star." No need to spend that kind of $. My personal favorites were the Sylvanias. That said, the 1980s GE and Philips are very good tubes and are still very available.
12AX7 and 12AT7 have notably different values. Subbing a 5751 for a 12AX7 is usually very safe. Subbing a 12AT7 for a 12AX7 may not be. But if you simply need to replace a 12AT7, many of the 1980s production ones are readily available at very reasonable prices. The Philips 12AT7, the Mullard CV4024, the GE 12AT7: all are very good. Personally, I think the Philips is extremely reliable and rugged
Groove Tubes does not manufacture tubes. They buy Sovteks, JJs, Shuguangs and others, test them, and then put a Groove Tubes logo on them. Mesa does the same thing - mostly using Shuguang tubes. The old Radio Shack lifetime warranty tubes were mostly left over Soviet military stock. The current civilian version of most of those tubes are today's Sovteks.
The Mullard reissues of today are actually made in the Sovtek factory in Saratov, Russia.
In trying a 12AT7 to replace a 12AX7, it sounds like you are wanting lesser gain. It is probably safer to get there by using a 5751 instead of a 12AT7. In terms of cost effectiveness, your best bet is probably to get your hands on a 1980s production Philips 5751 or GE 5751.
