I don't quite understand why it doesn't work if the CD player is totally turned off.
Turning the CD player off doesn't remove the components that preceed the output jacks. It's possible that there's a 10K resistor across those output jacks, and that resistance combined with the 100K pot on the Quickie would present a very low load for the Seduction to drive, resulting in poor low frequency response, and actually twice the cable capacitance that you would normally have!
For another example, assuming you have a Rega Planet, there may be a compensation network in the output too, which could add other undesireable effects if the Seduction "sees" these as part of its load.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg137.imageshack.us%2Fimg137%2F8739%2Fregaplanet2000012rq4.jpg&hash=0d5fc77baca11b2a561b04720798a7c95ef6c094)