I want to attempt a description of the "dark" sound. The sound is slightly rolled off in the upper registers and the all round treble energy reduced. If you have ever heard what a real British Mullard 12AX7 does to the sonics you get a decent idea of what dark is . In addition it is a full sound but not airy sound. That sound is typical of many older tube amp designs but are not what modern tube designs offer. So depending on your age you might conger up your Fathers 1960 EL-84 amp, if you have ever heard one you would have a good idea of what people are saying, when they say dark. Well I tried!
