The Mullard 12AX7 / ECC83 Reissue is a nice tube but in my opinion better suited to home audio than guitar amps. The tubes have well balanced triode pairs and a very even flat response. Compared to a Tung Sol it sounds a bit flat, but so does a NOS Mullard. Microphonics is not an issue despite the larger that average plate structure. The transconductance on my sample was the same as two NOS samples I measured. Not really high gain at all, but a real good noise floor and a nice smooth tone that doesn’t encourage ear fatigue the way some preamps can be.
The Tung-Sol 12AX7 is my favorite preamp for guitars because it accentuates highs and lows. The Mullard 12AX7 adds virtually no tone coloring and is smoother sounding to my ear than a JJ ECC83S. For hi-fi gear the Mullard will likely be a winner but there are better choices for guitar amps for less money.