You can use 12AX7 in place of either tube on More for testing purposes to verify that it will fix the issue, this will not hurt anything. Using 12AX7 in place of the 12AU7 will give a higher output volume, and using 12AX7 in place of the 12AT7 will just give a slightly higher output impedance.
I've had very good experiences with
Tube Depot. Unlike some distributors they actually test their tubes before selling them (a novel idea for sure), they have a large selection, and their customer service is great.