Before you spend money on a tube amp that acts as a processor, try an actual processor. I would suggest a BBE Sonic Maximizer. These gizmos alter how your system reproduces transients (via phase shifts and and relatively simple dynamic eq- it acts somewhat like an aural exciter), differently than a clipping SET amp for sure, but subjectively it's rather similar, surprisingly enough. It can also boost the lows and highs, similarly to how a high output impedance amp will interact with speakers. Pick up one of those for <$30 used or about $80 new at your local Guitar Center and throw it in a tape loop. Turn the 'lo contour' and 'process' knobs to about 11 o'clock, put on some music, and enjoy the resulting phat, punchy bass, sparkly highs, and image that seems to jump from your speakers. It's a striking facsimile of single-ended tube sound, without the limitations and expense. And you can turn it off whenever you wish.