That might be just variance in the horns as well. I haven't noticed any pitch problems with my Mack in the compensating range, or in its tone quality there. Of course, I do use a fairly large mouthpiece compared to many other amateur players. But I've used that horn without any difficulty to play tuba and bass trombone parts (with a Wick 3AL). When I got the horn, I tried a couple of the Mead mouthpieces (SM4...), but found them to be just too constraining. For me, in fact, it's pretty much of a toss-up whether it's easier to play the Tuba 1 parts in the Tuba Christmas book on that euphonium or on my EEb Wessex tuba. But the tuba just sounds better in the octave below the staff, where the euph starts to sound a bit more like a bass trombone.
I confess that it's difficult (without practicing on a really regular basis), to get the F and the E the full octave below the staff (Is that E1 and F1? Anyway, it's the pedal F on the F horn and the E a half step below it). How are those on the Wessex?
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)