Another possible solution that is fairly common is to put a splitter on the sub that gets hooked to both left and right inputs. (This is if you are using the line level inputs) This increases the signal the sub sees so the "auto-on" circuit kicks on sooner.

There are a number of companies that make a "Sub connection kit" that includes the Y adapter and all the cables you need.

I'll be interested to know if it helps you.

Michael