I've never had a problem with getting an air-tight seal with the paper towel roll and garbage bag trick. But I may not have described it sufficiently clearly. The paper towel within the garbage bag must fit very tightly into the bell and be forced down very firmly. Of the instruments I've tried it on, I've found leaks in only two: an old 60s vintage Martin baritone that leaked severely through the valves; and my 1924 Buescher Eb tuba that surprisingly (original valves and plating) leaks only mildly through the third valve.
It sounds to me as though your first valve is leaking. With the bell blocked, you shouldn't hear ANY hissing or detect ANY air coming out of the bottoms of the valves. I just tried the test on my Mack Brass euph and just about busted my gut before popping the roll (about a half-used roll in a plastic bag) out of the bell. Not a hint of a leak.
You really need to talk to Tom about this. It's possible that one or more of the valves was poorly fabricated, is cracked, or was ground or polished too much. This should be covered by the standard warranty.
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)