What causes "cupped hands"?

I've been dealing with this puzzling phenomenon for several months. I can't seem to rid my system of the "cupped hands" or megaphone effect. Acoustic treatments seem to help but haven't diminished it to an acceptable point. I've been told that cables can cause it but haven't been able to confirm this. The system consists of:

Mark Levinson 390S CD Player
Antique Sound Lab KMP-60 Fox DT Amps
Aerial Acoustics 7B Speakers
1.5 Meter ZCable LIVE v4 Interconnects
4' ZCable PASSION v3 Speaker Cables

Any ideas?