The only thing I can think might be the cause is maybe the other amps you have compared are sonically wider bandwidth (like a Blackface-ish fender tonality for example) where the Bass and Treble frequencies might extend lower (Bass) and also higher (Treble) than the Tramp. Since the Wah is really a Tone control maybe that is what you are hearing. Now that you mention it I always thought my Wah sounded better through my Blues Junior than either my Tramp or 5E3, both of which (to my ears anyway) are more tonally centered in the Tweedy midrange of frequencies than the Blues Junior, maybe that limits the sweep of frequency extremes that the Wah can produce when "filtered" through those amps and causes it to sound weaker?
Just a theory I'm throwing out there, I really have no idea.