Smooth and well balanced tone, with a wide tonal spectrum and a high brilliance. The TT 12AT7 is a great tube for guitar/bass amps as well as audio and HiFi tube amps.
Hint:
In many cases a 12AT7 or 12AU7 can be used instead of a 12AX7/ECC83 to reduce the gain or just to make the preamp a little bit gently. The biggest difference in sound will get attained with the 12AU7 instead of a 12AX7 – but at the end it is a question of taste.