I don't understand it either. I took mine out of the system for a few weeks thinking I could sell it.
But, just before accepting an offer I put it back into the system for one more time. Whoa, I couldn't take it back out!
I think if QOL would have added a volume control it could have been sold as a full preamp and would have been more in demand. Add on boxes sometimes are a pain.
Too bad though, great, great product!