ok... don't know about the GTXP so bear with..
on the Audigy there is an Audigy 'Surround Mixer' which has several parts (EAX settings, Speaker setup, and an Audio Mixer-much like the Wiondows Volume Control Setup)
In the Audigy Volume Control Mixer there are several volume settings - WAV, MIDI, CD Audio, Line-in, Mic, etc. The last setting is for Recording Volume Control.. Clicking on this will not only let us change the recording volume level but also the recording volume source(Midi Synth, Analog Mix, CD Digital, What You Hear, Microphone, Wav).. All we have to do is set it to Line-In to fix the problem you are having..
I know it's a different card but if you have a similar setup this may give you a hint as to where to look/what to change
hth
ripthesystem