This is at the front of my mind, because I have recently returned to valve/tube amps, having flirted briefly with a hybrid amp - a Marshall AVT150 - for about 9 months. The reviews of the Marshall were mostly ecstatic, and it sounded fine in the shop - but some home use, and a bit of gigging and recording have found it wanting - the OD channels just didn't have the warmth and crunch I got used to in past amps (which include a Fender Deluxe Reverb, a Laney 30 watt valve combo and a Fender Blues DeVille)... And what did I go back to? - a Peavey Classic 30 - followed more recently by a Fender Hot Rod DeLuxe, and even more recently by a Peavey Delta Blues - all of which have pretty much all valves in the audio signal path, although in all cases the reverb and effects loop drivers are solid-state. Nonetheless the Peavey and more especially the Fender have a 3D quality to their sound that that puts the AVT to shame. I feel reasonably convinced that this is due to the valve/transformer output stage.
Comparing these amps, highlights another interesting issue - I experience the Peavey and the Fender as much louder than the AVT - this despite the rated output of the Peaveys being 30 watts, the Fender 40 watts and the AVT 65 watts. The AVT also has an inherently more efficient speaker cabinet design. So what's going on here - are Peavey and Fender rating conservatively? - Marshall optimistically? or do the speakers have very different efficiencies? Or, as seems more likely, do I like the sound of the Peaveys, and the Fender so much that I am incapable of being objective?