I spoke to Mark about the Metro loop and he likes it. I would love to try the Merto loop as well, but like Nick said, I would hate to change anything inside the amp.
I totally hear what you are saying there. I tried to get mine to work in my self built slo type circuit and it didn't work for some reason (cut the top end grind off and knocked the treble down). Could have also been just the fact I did something wrong as well. Anyway if Mark likes it and works with his amps I think I may go that route.