Just tried mine with a few valves.
The Sovtek 5751 sounded good. It's meant to have less gain than a 12AX7 which sounds about right, but the whole little amp sounded louder with that in.
I did stick in a Sovtek 12AX7LP (similar to a JJ ECC803S with the longer plates) in there and it actually sounds killer. Playing it through a 4x12 with V30's. It's actually mindblowing.