Well, I think that depends on just what Line 6 gear is being used. They have a pretty wide line, and some is very good, and some is quite crappy.
Most of the Line 6 stuff people end up seeing and hearing is the budget end, and well, those are quite poor (Even the 'Bogner Designed' end of the Spider line).
They also have some very outstanding pieces of gear (Notable pieces I have used are the Flextone II head and cab, and DT series amps, as well as the Pod XT and HD series). I have to admit though, even with their higher end series, I do not care for their combos, but then again, I do not care for the combo (open back) sound myself.
Do not get me wrong, I *love* the ADA tone, but are you comparing apples to apples, or an apple to some piece of fruit flavored cardboard?
I have had and used both (ADA and Line 6) in moderately large venues (Well, actually, large a few times in the 50k people+ range) and they both hold out well.
But, to compare an ADA rig (Even priced nowdays, used, is *lot* more expensive than most Line 6 combos - their budget lines) is rather unfair. Even Marshall and Fender make some really crappy amps in that range.