@ Actusreus...
the setup has the main speakers and the sub receiving the same signal. this signal is high passed, ie, the low frequencies are gradually rolled (80 hz, 6 db per octave) between the preamp and the poweramp. the main amp and the main speakers are therefore relieved of the lowest octaves. the amp in the vandy sub then boosts the signal to recover what the high pass filter removed - it's incredibly effective and the most seamless integration i've ever attained with a sub (velodyne, jl audio, vandersteen) and mains (wilson, revel).
