The reason I asked the question was, I did find a Sub useful when I used to run my merlins with a sugden A21, but the moment i upgraded my amp to a leben CS600, the bass just improved and I did not need it anymore.
But if a sub is needed i would recomend a good Sealed box sub..a REL strata is great and so is an MJ acoustics Ref 100
Bobby, I changed my speaker setup to a corner placement and i find the Bass very tuneful and controlled so the placement factor also needs to be considered.