Krell 400 XI

Please have me advised. I recently bought a Krell 400xi new from a local dealer.The mid and the high are impressive. The sound stage is fairly good comparing with some of the MF I used to own.The only disapointment is I cannot get the bass right. It is not boomy just not defined and very weak. We are talking abt Krell attack here but I could not find any "attacks" at all. I use a small 5" two way book shelf speakers. I could not find any dedicated sub-output but a pair of pre-out. What is the best way to hook up a sub-woofer to this intergrated amp.since I am not ready to purchase a full range speakers yet. Thank you.