Best sub hook up for 2 channel- rca or speaker?

I have read in mapleshade and heard from a martin logan rep that speaker level hook up sounds better than rca and makes for better integration in a two channel set up. This baffles me somewhat because the amp board in the sub has to have a hi/low converter to drive the amp and thus we would be converting the signal when we might not need to. I have a friend who tried it and said it did sound better and I am going to try it on my logan sub as well but I am more interested in why this seems to work-and if anyone out there is actually doing this. To clarify the hook up- 1.amp to speakers full range. 2. amp to subs hi level inputs, four speaker connections instead onf 4 rca's...Thanks for any responses...Scott