Why do we want to distortions?

Interesting. A few years ago I made a prototype mosfet amp with adjustable output impedance (or damping factor if you will). By using adjustable current feedback. Damping factor of 20 was preferred with my speakers. I tested with DF 30 down to negative output impedance. Just to see how the sound was affected by this. But it comes down to what the speaker was design together with. I also did a spice simulation model of my speakers to be able to simulate the behaviour with respect to source impedance. Used this for some simulations of a current amplifier together with the speaker simulation model. However, my experiments using the MOSFET (class A voltage gain and push-pull output stage and some global negative feedback) did not show a strong correlation with tube sound. There where some to changes in the base as the base driver gets a bit more freedom. But this was not a scientific analysis, and it was only tested on my setup. I could not increase the DF over 30 in the design.

Both Cheever, Pass and several others agree on that to much negative feedback create high order harmonics that we are very sensitive to. Cheever also discuss aural masking and the effect by adding such distortion to the playback system. But does this masking effect also apply to to some extent to intermodulation distortion?

Intermodulations do not get masked. I agree with Cheever and Pass on this- but only to a certain extent. If the feedback is creating higher ordered harmonics there simply isn't enough of it. That is why I say you need in excess of 35dB. With that amount the amp will have the ability to clean up the distortion created by the feedback itself.

Thanks atmasphere for clarification about IM.

I agree with you that either very much feedback to clean up high order distortions or less than perhaps 10dB or so. There are different types of feedback and I’m referring to traditional "loop" feedback. Degeneration is a bit different. But I think error correction is much better if that is possible in the design. What is your opinion about that?

 Tubes do add bloom

Tube amplifiers sound different than solid state but are much more particular with speakers than solid state so if you want the tube amp experience to be as good or better than solid state you need a pre 1965 speaker to play on it.
 Repo

OMG really?  Maybe the ones you owned or heard but not always true.

Some months ago I listened on wonderful tube amp sound from Engstrom Arne (Swedish highend) as maybe one of my most memorable listening experiences with tube amps. The new Marten Parker speakers where used. But I still think you are right. Tube amps in general should work much better with speakers at my age... or older as speaker impedance can be very low these days.