Thanks for that guys.
Seemingly because the Pass is not grounded at the speaker terminals (floating earth, I think is the term, something to do with the balanced design), it will upset things by the sub only being grounded to one terminal(or not as the case may be).
The REL manual mentions that in some cases the sub will hum if the power amp is switched off.
This was the case when I hooked the REL up to the PASS - I simply switched the sub off before putting the Pass in idle mode. It seemed to work just fine and caused no problems when the Pass was powered up.On taking advice I decided it was best to leave the sub out of the system, a shame as I am late to the fold but really think a sub opens out the midrange when used sparingly. I am not a bass freak but given the REL brings more to the table than the Pass, I wish I had known this in advance - think I'll stick with my Nu-Vista and the REL - I like an approximation of real music, not really interested in an initially 'hi-fi' sound which palls greatly on serious listening.