I think part of the difficulty with gauging the strength of the A League is the salary cap and lack of relegation. The MLS is probably the closest comparison in terms of league structure. I've seen a bit of the MLS over the last few years and I think the best teams there are better than the best A League teams but the middle of the road teams are roughly the same. Glory or Victory definitely wouldn't be disgraced in the MLS.
I also think that the standard of reffing lets the A League down. Too many cynical challenges are ignored or not carded, so skillful players have less opportunity to influence games and teams can't string flowing moves together as easily. I think that makes the A League worse to watch that's it otherwise could be. If refs were consistent with yellow cards for repeated infringement or professional fouls then teams might have to play cleaner and the games might flow more.