I'm going through that very thing right now. I'm using a Mapletree 4A SE tubed pre. It's a two chassis pre. It can use either 12SN7 or 6SN7 tubes(switch on back)and is tube rectified. The sound is much more relaxed and the soundstage is wider and deeper. It does have drawbacks only one input other than phono and no remote.The one input can be taken care of through a multi input switcher but you are adding more connections to the signal path. I can live with out the remote as the sound is much better.More info here:
http://hollowstate.netfirms.com/#preamps
http://hollowstate.netfirms.com/#preamps