Hi Murph,
I've been following this thread and I suspect that the audible variations you experience are all a product of wave theory--that is, stationary standing wave patterns with nodes and anti-nodes that are formed by the low-frequency test tones. By way of illustration, if you imagine a long taut rope that you set into motion, you'll see the wave patterns and standing waves vary and move along the rope as it continues to oscillate.
Another contributing factor is your stereo hearing. Inevitably there will be ongoing cancellations and reinforcements of low frequency tones as they reach your separate ears through different path lengths from reverberation and reflections in the room.
There! Now I have to rest my brain and have another late-morning coffee. . .
Regards,
Alan
