Ummm, no. You can only use subwoofers on the .1 (LFE) channel. The other seven channels are for speakers only. You can split the LFE signal and connect several powered subwoofers to it though.
I was just curious as if you could use the 6th and 7th channels to power a non powered sub...
If the answer to that is no, id be curius as to why. I believe some recievers allow those channels to be used for a second room, It would seem with a simple crossover those channels could power an 8/8 dvc sub fairly easily.
Yes and no. You sort of answered your own question, in that these two channels would be intended to be playing from a DIFFERENT source than what the mains are playing. However, if you simply split the signal from the main input, theoretically, it should work.
Two things though: You will now be drawing more current from the amp section of the receiver, which will reduce total available power (headroom). As well as I don't think you will be able to use this setup as an LFE channel, as it will be a full range stereo signal (all that I have seen anyway).
I would say it would be easier to spend the $100-200 and get a plate amp or cheap used receiver for the sub.