There is, in theory, no difference between pre-outs and line-outs, so I cannot understand your question. You need to use a volume-controlled output and, if you are using a stereo source, you need to combine the two channels for the sub. Many subs actually have L and R inputs, thereby eliminating the need for a Y-connection.
So, be much more specific about what you have and what you want and, perhaps, we can help.
Kal
So, be much more specific about what you have and what you want and, perhaps, we can help.
Kal