Is this clipping?

I agree with onhwy61. Furthermore, some equipment cannot handle intense complex passages. A lot of single driver speakers have problems with that. Quad ESLs for instance, NOS DACs too.

If it sounds great at any and all volumes then there is no reason why it cannot play rock. If it is clipping on rock then it will clip on jazz too - if you play loud enough.

Well, I throw this out as possibility. Much Rock music tends to have plenty of sustained high volume. Much Jazz music tends to have more dynamic contrasts (some big band excepted). Many amps are capable handling brief power peaks, but are strained by continuous demands.

PEople are sometimes under the misconception that rock/pop music is the easiest to get right. When it does not sound good, the recording is blamed. I have not found that to be the case. Large scale classical and many rock/pop recordings especially newer loudness wars recordings were some of the toughest for me to really get right. The thing these have in common is longer high energy passages that get choked out in an underpowered system. Also smaller drivers may be challenged to deliver full dynamics with these as well. You may have a double whammy going on in your case with the low power tube amp and the single full range drivers. The recipe for good rock and pop in general is usually lots of power and larger or multiple drivers.

Mapman, most modern pop recording suck! The classical recordings are not subjected to the same mass marketing recording engineering aberrations. People in the know are not mistaken! It's the recording, not the amplification. Even those with power to spare suffer through most modern pop recordings.