>>106256508
It depends on the stage where the tubes are used, most cheap tube amps have them only on the preamplification stage, and the rest of the things is done with solid-state parts. With fancier amps you also get a power stage amplification done with tubes as well.
I have a small pentode JAN6418 amp and what it does in the tube mode (can be bypassed) is introducing bass rolloff and a lot of 2nd harmonics (1%+ at 100Hz, peaking around 4% at 20Hz). Yes this distortion is audible but I can't say it makes things sound better.