Seriously why aint' that stuff there?   The XT was the pinacle, any else since has been lacking and why I dumped L6.  The best thing any L6 owner can do is NOT use the X as sound card, but instead have a real sound card and use X via the analog outs.  You will not regret it once you figure it out and you can swap in any gear and record via analog instead of that usb interface stuff.  Free yourself of that USB L6 connection for recording.  Go ahead and use it for editor or whatever, but ditch it for recording purposes.  You will benefit in the long run.

I haven't tried Don's method personally, but as long as you cut the direct link between the HD500's input and output you effectively force everything through the computer based DAW before hearing the computer processed sound from the HD500's output.   (OK that's a bit crude in description, but it's similar to MIDI keyboards that allow Local Off thereby forcing all keyboard output to go out via a MIDI out port through a computer or MIDI sequencer or whatever and then back in to the keyboard's on-board sound module so you can hear the results of what you're playing).

This is the important bit:

-Launch the Line 6 Audio-MIDI devices menu, and under the inputs & recording tab please decrease the hardware monitor level all the way.

That turns down the direct signal path link (Local) in the HD500 itself (only affects it whilst the HD500 is connected to a computer.  I have used the HD500 in the same way with POD Farm 2.5 and it works.  But as I say i haven't specifically tested this with a DAW and using the POD HD500 for re-amping, however Don's suggestion looks correct to me on paper and it should work as far as I can see and you should be able to monitor the results of your re-amping as you record the replacement track

Nick