In-Line Encoding Vinyl to MP3

Last night I hooked my brand new IAudio U2 up to my buddie's amp using a cheap rca-to-mini cable. The IAudio U2 can encode mp3's on the fly and so we recorded about ten songs off of his vinyl collection onto the tiny IAudio player. I believe the rca-to-mini cable was connected through the tape-out plugs of his stereo. All seemed to go well. However, after plugging the IAudio into my computer via usb, the files sounded extremely bass heavy, the levels were way too high and the mp3's were basically unlistenable and useless. How, using the IAudio U2, do I correct this and enable on-the-fly mp3 encoding of vinyl to mp3?
Could the output level have been too high. Or, did you have the input level too high on your IAudio U2 (if it has one)? No other explanations really jump out at me. You could check the spec sheets on the output level of his amp and the input level of your U2. Good luck.