I wouldn’t say the Mullards are dull. During all these years Mullards made many different types of 12AX7 or 12AU7, and many others.
The 50’ Mullard is not the 70’ sound. The short and long plates doesn’t sound the same also. Same thing for the Military specs etc etc etc.MC1, MC2, MC6, I65, I63, I60, CV... , M..., All have ther own sound. None ot them sound bright or ananlytical (I think).
The way you melt your different tubes in your gear is the key fot obtaining the sound you like the best.