The Sigs will do justice to some fine quality gear so the point-of-no-returns is likely elsewhere in your system. Vandersteens are very sensitive to toe in, tilt, and distance from walls but "falls to pieces" on complex works sounds more like a speaker control (signal) problem as noted above.
Vandersteen
After hearing many good things about Vandersteen speakers I purchased a pair of 3a signatures. They sound beautiful with chamber music or small group jazz but quickly fall to pieces with symphonic works or rock. Have other people noted this deficiency with Vandersteens?