The Krell should work fine and for your size room I would definitelly recommend a sub, possibly even two. The M-L Descents or any of the mid sized REL subwoofers would work very well. You should also read a recent post on M-L stats by Audiokinesis regarding potential problems with tonal balance.
Amp for Martin Logan Ascent
Which Amp would you guys recomend for the Ascents. What do say about the Krell 400xi. This is what I was told by a friend. Also would you say that it needs a sub woofer and if so which one? I listen to all sorts of music from the rock of the `80s to jazz and pop. Also my living room is pretty big (About 35 feet by 25 feet).