I found that the problematic threads on my Wessex tuba "broke in" too -- but I think (from looking at the threads carefully) that this amounted to some "filling in" taking place with the silicone paste I used on them as lubricant. This eliminated (or at least reduced) the tendency to cross-thread when tightening them. One must continue to take care. I'm a bit surprised that at this point Wessex hasn't absolutely nailed the threading QC issues. If I could take the threads on my 1924 Buescher tuba and transfer them to my Chinese instruments, everything would be GREAT. Of course, that would also require transferring the harder brass alloy as well.
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)