sentences:  Ampeg continues to manufacture and sell Heritage Series and SVT Pro Series models of SVT.The SVT amps with 6146B tubes tend to put out a bit more power as well as have a more pronounced grind in low mids, as opposed to the more round, deeper bass sound provided by SVTs with 6550A tubes.This sound characteristic is mostly due to 6146 being much lower in transconductance and less sensitive to drive signal, requiring a higher level from the Pre-amp, which can create more "harmonic growl."