I mainly use the art unit as a mic pre (to amplify my tuba over my amp) and for the mic it is pretty clear, transparent and noise free. I frequently use the unit and absolutely love it, especially for the money. On that basis I always thought the tube was just for mood lighting and exchanging it for another tube would be a waste of money.
I have recently tried it with a bass (active and passive) and with the bass it did sound a bit weird. Tubey, but not nescessarilly in a good way. Not a eq thing, it might warm the sound up a bit, but not much, not like a fishman platinum pro. It is hard to describe, more like a timing effect or a rubbery feel. I assumed it had something to do with the interaction with the tube pre in my eden amp. In my case the noise levels were way below noticeable at live volume though. Maybe the noise problem is a quality control thing and differs from unit to unit.
yamaha club #34, yamahe BB club #23, lefties who play righty club #192.