Ok, so you have discovered the inevitable phase shift issues when trying to blend subs with bass reflex speakers getting a full range signal. Even the 60hz setting you tried is probably too low to avoid the problem. Why not follow evidence-based best practice, and set the crossover an octave above your speakers f3? Try 80hz, recalibrate, and give another listen. You'll very likely get a more seamless blend with the subs, reduce modulation distortion, and, however slightly, ease the power demands from the AVR.