what are you trying to record? are you trying to record a guitar with a mic or are you using a guitar with a pickup? is the sound you are wanting to record coming back to you through speakers or headphones?

if you are using a mic are you getting the sound from the speakers rerecorded causing the slapback?

i record in RB with vocals and a guitar and although i use a different interface maybe i can help if you give some more details.