In my (limited) experience, I'd say that the volume delivered by a valve amp is more 'linear' than SS or class D. In other words, 8 is much closer to twice as loud as 4. Class D amps that I've had tend to get more 'shouty' without getting much louder whereas valve amps keep the inherent sound, albeit with a bit more drive, as they get pushed harder. These are my layman's terms which might work for some.