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I may be wrong, but I think some tube amps don't like to have no
speaker (or equivalent load) connected, so that might be a cause for
your distorded signal. I think it would help if we knew:

1- Do you talk about a normally clean sound that ends up distorted or
2- A fuzz sound that sounds way too harsh.

The 1st case is a problem. The second one is normal as you remove the
speaker sound from the equation.