flat5

Certainly the tube amp will have more distortion. Great care with the output transformer and circuitry is required and that is a big part of the extra expense of a good tube amp. The output feedback network, and balancing can make a big difference here.

supafly, you don't mention non-harmonic distortion tests, such as weird mixing products. You should do two tone tests too.

other tests you might consider:

Test freq. responce at just under clipping.
Pulse tests might be interesting. (overshoot, under shoot, sorta like slew rate, I guess)

You might test the noise floor at zero volume and full gain with input terminated.

What will be the dynamic range before clipping with a stated noise floor?

How critical will the load impedance be? (test at 2 to 16 ohms in 2 ohm steps at half power, for instance) test damping? various distortion tests.

just some ideas to keep you busy :-)

I wonder what tubes you are using? what circuit? point to point or circuit board wiring?
reply if you want to.

I would think the output transformer, a very good power supply, some low noise tight tolerance little components, careful feedback network, and common ground point (or lack of loops) is the big deal here.