The Willson big-bored Eb and the Bessons/Booseys are indeed very different. But you learn to deal with the "stuffiness" by slow air... over blow 'em or try to force anything and they'll respond... unfavorably, with poor intonation and awful response. I've mentioned this on trombone fora as well... slow air for smaller bores, not high-pressure air.
The Willson is a nice horn, but I'd be loath to play some pyrotechnical stuff on a 5-valve system; for me, the "double tuba" configuration is just super facile for that.
To each their own

If you like the big valvers, the Meinl Weston is a nice beast too; I'd love to try the rotary version.
Instructor of Tuba & Euphonium, Cleveland State University
Principal Tuba, Firelands Symphony Orchestra
President, Variations in Brass
http://www.jcsherman.net