To answer a few different posts and provide some info...
I have a JBL 8400P sub and it has both L/R line level inputs, 50-150hZ variable crossover control, phase switch etc.
AES3-MKII preamp (made by Cary) which has 2 sets of L/R line level outputs.
It is feeding a pair of modded Dynaco MKIV monoblocks. (Must give grannyring some props here. He rebuilt/upgraded these things, and they are beautifully done, as well as sounding incredible. Really nice guy too, as others have said here.)
The JBL manual gives as an option putting a Y inline if your pre-out is only a single RCA jack, so I don't see how it could harm anything to Y the connection on both ends, as stated previously it is mono.
I have a large volume room with hardwood floors, which makes my sound a bit on the bright/thin side with some recordings. I am just wanting to tinker with the sub to see if it makes the bass deeper without being too muddy/booming, and I didn't want to dump a fortune into cables (it's about a 10-12ft. run from my rack to the sub) just to find out that it was not much better.
Thanks for the feedback so far.