I think Dave definitely has this right. In addition, I think what we may be seeing here is partly a matter of the "British sound" as opposed to, say, a more "German sound". You definitely will get a different tone quality on one of those big compensating Eb horns than on a non-compensating German or American style horn.
But it's not a consequence of the compensating system -- at least in this case. The range in that piece is pretty much solidly in the middle range of the Eb tuba. In that range, the compensating system should be having no (or no discernible) effect on sound. It's not in play. It's not being used. If you look at what he's doing as you're listening, he's fundamentally playing a 3-valve horn. I think there may be a few times when he uses the fourth valve when he dips into the stuff below the staff, but his hand is hidden there. Otherwise, I can't see him using the fourth valve at all.
Also, this recording was made ... what? ... over 25 years ago. You need to wonder about the fidelity of the recording. And those horns have 19" bells on them and pretty pronounced bugle taper. They can pitch a lot of sound if desired. (That's one reason I'm planning on replacing my BBb with one .)
Anybody notice what I regard as a weird technique with the third finger? He uses two fingers on the second valve except for when he needs the third one. I suppose I can imagine some advantage to this, but there seems to be a cost in continually moving the third finger back and forth from the third valve -- kind of like the habit many trombonists have of checking (particularly third) position by touching the bell with a finger of the slide hand. Do a lot of people do this?
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)