I have the PC38X and the GC7. That headphone definitely distorts in bass heavy movies for me with the the GC7, but not from other sources. Especially with SXFI on. The "budget" Koss stuff, too.
I just assumed it's because of the 10 ohm output of the amp. I have to turn up the volume to unhealthy levels with the 38X/Koss to hear any impactful bass, even for regular stereo.
You might want to get a higher ohm Sennheiser 6X0, 560S, or 58X to pair with the GC7 instead.
Unless we are both dealing with dud units (the GC7, that is).