I have a couple of Electro Harmonix English Muffn pedals which are the core of the bass overdrive sound. This is a fantastic sounding pedal and is really a 'proper' tube preamp in disguise - has enough level to drive a power amp, proper 18v supply to the tube. Compared to most if not all solid state pedals I've tried its much more 3 dimensional, ballsy and flat (no bass rolloff).
Its possible to easily pull the 2 tubes out (it comes with 12AY7) and put others in. So, essentially out of curiosity and to get higher gain for guitar work, I order a pair of EX gold 12AT7 and Tung Sol 12AX7s.
Quite a difference. The AT7 has more gain and a sharper more forward presentation. The AX7 has (as one would expect) the most gain, and even more cut in the upper mids. By comparison the AY7 is much flatter or duller sounding - and even with eq, a different presentation/sound. The AT7s are enough gain for the spector I use (that has very hot EMGs) and the mid structure is an improvement in a mix situation so these are keepers.
Cranked, both AX7's with medium output guitar humbuckers give classic super saturated sounds for metal with plenty of 'chug'. Noise is a bit of an issue. Not that I really use that tone on CA albums but its fun for funs sake.