Is this clipping?

Psag,
If you would read the Simon Thacher of Spectron paper as I suggested above then you would see easy explanation why you can "drive" jazz but not heavy rock

EASY: your amplifier's power supply can reach headroom (peak) voltage and current required for jazz AND heavy rock (for your speakers, size of room and volume you set) but but but not for the duration (duty cycle) heavy rock requires.

If you want to listen heavy rock well - change your speakers to more efficient or amplifier with better headroom (do not confuse it with rms power as its not the same and power in rms is probably the most misleading specs in audio as it tells me nothing but some clue...).

Your dealer or manufacturer must be your consultant: they do not sell refrigirators and got to work for their money !

Dob,
I've read that paper and I agree. The 300B amp can reach the peaks, but can't sustain them.
Fortunately I have Spectron monoblocks in my other system!

"The 300B amp can reach the peaks, but can't sustain them. "

That sounds like clipping to me?

Thinking more about recent digital rock recordings, seems to me that people make the mistake of associating compression and loudness with a poor recording. I disagree- they're supposed to sound that way. Unfortunately too much of the current audiophile equipment is unable to reproduce it well. This is one of the reasons we hear so much Norah Jones-type music at audiofests.
Last night I tried to listen to Katatonia-Night is The New Day on my 300b/single driver system, but simply unlistenable. Listening today through headphones- fantastic!

Psag,

I think it is a safe thing to say that it is generally harder and more expensive to get a system to sound really good when played (realistically) loud than unrealistically soft.

Louder requires more power than soft. That is pretty intuitive, right?

Well, a lot of new recordings are louder overall, so more power than ever is now required as well.

Efficiency helps. Suitable degrees of efficiency overall to deliver loud recordings without clipping can be achieved readily these days with a more efficient amp, like Class D, and/or more efficient speakers.