Karl- I JUST bought a Pass Labs X-250 to go with Martin Logan speakers and an REL Storm series 1 sub. It replaced a Bryston 4B-ST. The Pass is a fully-balanced, differential amp as is yours, connected via XLR Shunyata Aries interconnects from my Anthem preamp. The Storm is far more limited in it's input & adjustments then your Stentor, which may mean another upgrade is on the way (a Stadium III, maybe)!
Anyway, I started to get a bad ground hum from the REL when it is connected to the Pass via high-level Speakon connection in the 'traditional' setup method in the manual. It was most noticeable with the amp in standby mode. Connected in this manner is supposedly very bad for the amplifier inside the sub. This, of course, does not apply to the line-level LFE input.
I talked to Jim at Sumiko, who said to move the Speakon connector to the Hi-level BALANCED input, but ONLY connect the red & yellow wires to the positive speaker connectors. The black wire then attaches to a good chassis ground on the amp - not the right channel negative as normal. This will put it back into summing mode - I Think - I haven't had enough time to get my system back together completely yet to test it fully with the new amp.
Did they mention or have you tried that combination?
Hope this helps & Good luck.
Bob Houser