Most methods of measuring unity gain are flawed. There really is no such thing with modern chipsets and designs.
It's all about what sounds best.
Back to the OP's problem.. It should be fairly easy to reproduce and verify the problem by using a bass heavy source and playing it back into the tb and the m10 combo, and also to just the m10. The distortion should be easily audible on the headphone out. Once reproduced, you can try and make adjustments to improve the situation. If you have a voltmeter, you can measure the signal level on the a/c setting. It won't be entirely accurate but if done properly it can be a useful reference.
Unless my quick measurement was wrong, the sample that was posted had an average rms level of -23dB. That's extremely low and it should have looked low on the m10 meter.