The problem (I think --if I understand your set up correctly) is that if you output from the preamp outs on your amp to the sub you will not have control over the volume in proportion to the level you are playing the amp at.
Connecting a sub to amp pre-amp outs?
I know this is a painfully ignorant question for most of you guys, but bear with me. I have never had a sub before so... There is no sub out on my integrated amp, but I do have a pair of pre-amp out jacks. Can I connect these directly to the low level inputs (L and R) on my powered sub? The sub manual says I can do that, but my Arcam amp's manual doesn't say anything about that--in fact, the manual more or less says that the only time I would need the pre-amp outs is to add a power amp for bi-amping. Right now I am using the second set of speaker binding posts on the amp to run regular speaker cable to the high level inputs on the sub. What will I gain/lose if I can go from the pre-amp outs on the amp to the low level inputs on the sub?