Thread Focusrite 6i6 - S/PDIF issue/question

Just purchased a 6i6 and I am loving this interface -- so much better than the Steinberg I was using.

Here's my issue: I'm using a Kemper guitar amp modeler that sends out not only my guitar signal, but also MP3 backing tracks that flow into the Kemper via my computer. I want to be able to record as:

Kemper (guitar + backing track) -- > 6i6 ---> Reaper

I've had very good success with:

Kemper XLR --> 6i6 XLR

However, when I use S/PDIF, all I get is the Kemper guitar signal -- the backing track doesn't register in MixControl.

When I'm using XLR, the backing track moves the indicator bars in MixControl before I start strumming. But with S/PDIF, the bars don't move until I hit my guitar, and not surprisingly with S/PDIF the recording in Reaper has no backing track.

Why not just stick to XLR? Well, the XLR recording is awesome, but the S/PDIF recording just has that little bit more oomph -- seems like the bottom end is a touch beefier, which is important because I'm playing droptuned Sabbath songs.

Any idea why the 6i6 isn't picking up the backing track when I use S/PDIF?

I'm using Windows 7, if that's important. When playing, I monitor with headphones plugged into the Kemper

Thanks for reading!