Kid, a correctly-designed tube amp can be just as transparent(flat frequency response, inaudibly low noise and distortion)as a well-engineered solid state design and neither will audibly affect the sound, except to amplify it of course. One of the illustrations of this was the blind tests years ago by Stereo Review, one result of which was that a $6,000 pair of tube amps was able to match the performance of a $200 solid-state stereo receiver and couldn't be distinguished from it. However as Alan has pointed out, some tube designs have problems with uneven frequency response due to their high output impedance following the swings in speaker impedance. Another problem that can arise in the low power tube SET amps(e.g.5-10 watts)which can sound clean at moderate levels is the second harmonic distortion which they can exhibit when forced to high output levels. Sometimes tube enthusiasts attempt to use attractive terms such as "warm" or even "musical" if these problems arise, but an amp isn't supposed to be a musical instrument.
