The 5751 has nearly the same topology of the 12AX7 (see below) - both of which are higher gain than a 12AU7. Be careful when selecting tubes for this reason - just because the sockets are the same and these two tube types can
be interchanged, their topology may not be the same (due to different plate voltages, etc).
Here's a gain comparison chart for such socket-types from Torres Engineering
Gain comparison chart:
Here's some more from VacuumTubes.com
This is the "milspec" version of the 12AX7, but is also slightly different. The 5751 has the same plate resistance as the 12AX7, but the gain factor is only 70 compared to 100 for the 12AX7. The 12AT7 also has a gain factor of 70, but since the plate resistance is different, the 5751 is closer to the 12AX7. The older "real" 5751's have extra thick mica spacers and extra support rods to minimize microphonic response. 5751's also have matched triode units. Late versions produced by Philips/ECG don't have the extra support and don't perform as well as the older 5751's.
The earliest ones were all made in the USA by the usual manufacturers like RCA, Sylvania, GE and Tung Sol. Very early ones have rater large size plates. The "plain" version without the "A" suffix can only be used in parallel filament circuits, not series. This usually isn't a problem as series circuits are only normally found in TV sets. All of the tubes will have odd voltages other than 6 and 12. If all of the tubes in your set start with 6 or 12 (except for the 5 V rectifier) , then it is a parallel circuit
12AU7 / 12AU7A:
The 12AU7 is a medium mu multi-purpose double triode in a 9 pin miniature package. The only difference between the 12AU7 and the 12AU7A is the "A" version can be used in series string filament circuits due to it's controlled warm-up cathode.
If you want to replace your 12AU7's with similar gain tubes, I'd suggest looking at Amperex's 7316 or CBS/Hytron's 7730. On the other hand, if you want to experiment with higher gain (more headroom) tubes, check out the ever popular NOS Amperex "Bugle Boy" 12AX7. But I'd first recommend you ping someone at Manley and get the opinion right from the horse's mouth - they know their tubes, believe me. EveAnna is also a moderator on the Geekslutz forum, so you might want to post your question over there.
Tube retailers: The Tube Depot
(I've ordered from them - they are incredibly nice and knowledgeable, and will match tubes at no extra cost). Vacuum Tube Valley
(never ordered from them, but they have an incredible stock list and a very informative website.