With guitarists it's all about the tone of a tube in it's overdriven (non linear region) state. What happens is that the original signal gets softly (ideally) compressed and in changing it's waveform shape, harmonics get added or removed from the original signal. This is the reason guitarists like to piddle with tubes and like to come up with their own unique voicings. In effect, what you are really doing is tuning the amp as an instrument much like you would a guitar by changing pickups, strings, etc. For bass (and there are a number of very heated threads on TB about this) in my opinion, many players like a slight amount of overdrive in their tube amps (others don't, hence the Tube vs SS discussions) but in the end we bass players really seek ample power and headroom. This will come from the circuit design considerations of the amplifier and although the tube selection will make a slight difference it is not paramount to the overall sound as it is with guitar amplification. This is why we don't play with tubes the way guitarists do.
Make sense?
