The common factor seems to be the velodyne then as i have a DD12.
Since my DD-15 shows the same behaviour when connected via XLR to my SP-1 PRO, this seems a possibility! I haven't connected the sub using the unbalanced connection, though.
The thud is there every time the relays in the SP-1 switch. Considering that this happens a few times on my unit when I watch a DVD (e.g. when the digital formats change, the processor does the relay blues between analog an digital inputs...) this is not really adding to the movie experience.
Has Bryston ever investigated a thud-issue in conjunction with Velodyne subs then???