I've heard various 12AX7s and 5751s in my own phonostage (Viva Fono), and that of two friends (a Tron phonostage and Hovland HP200). The closest to a "universal" 12AX7 (sounds good in all applications) was the Amperex Bugle Boy. That delivers a warm sound, but is not overly muddy and lacking in speed and dynamics. The Mullards tended to be mixed bags--in some applications it sounded phasey and muddled. 5751s are also reasonably universal, though they have lower gain than 12AX7s (about 70% of the gain of a 12AX7).
The tube I settled on for my Viva Fono phonostage is the Telefunken ECC803S. This is NOT a universal tube. It is extremely fast, clear, dynamic and tight sounding -- probably too much so for most applications.
The tube I settled on for my Viva Fono phonostage is the Telefunken ECC803S. This is NOT a universal tube. It is extremely fast, clear, dynamic and tight sounding -- probably too much so for most applications.