pre amp tubes

They are physically interchangable, being 9 pin minuture dual triodes. They are not electrically interchangable however, so you should probably refrain from substituting one for the other.

If you search the forum you will find the question being asked over and over again, and hundreds of great suggestions as to which is the best 12AX7 and 12AU7 type tubes.

IMHO, it is also very context dependent so you should search for suggestions for the tube types that work best in the specific gear you are using. Without knowing this the question is a bit like asking what the best tires are without knowing if you drive a sports car, light truck or tractor.

The heater current of a 5751 is slightly higher than a 12AX7, be sure that your power supply can take it before making this change willy nilly.

Additionally, the gain is 70 vs. 100 for a 12AX7, so your signal to noise ratio may improve, but the lower gain may be a compromise in certain systems.

12AU7 and 12AT7 have no place being substituted for a 12AX7 as they are electrically dissimilar. However if you are more competent than the designer of your circuit, by all means have at it. You will do no damage.

http://bitsavers.trailing-edge.com/components/rca/1961_RC-21_RCA_Receiving_Tube_Manual.pdf

And if your circuit is a hybrid tube/transistor Chinese designed circuit based on op amps, you may very well be more competent than the designer of the circuit.