Although the amp, most likely would work, with 5 of either type, it would be best, to run the amp, with the tube complement, that it was designed for. The gain, of a 12AX7, is higher, than that of a 12AU7. Most likely, the gain stages, depend on the tubes, to be a certain gain, to produce the intended sonics. It shouldn't hurt to try, but most often, it will sound better, and, as the designer intended, if you use the tubes, it was designed with.
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