I own a FiiO E10K OLYMPUS-2 and may be able to offer some guidance.
The FiiO E10K Amp & DAC is very simplistic in nature. I'm assuming you have read through the short guide and downloaded the updated firmware - available on FiiO Support webpage (You will need to have an account and product registered to receive update).
With that all said, you may want to try the following options below to improve sound (Keep in mind the FiiO E10K Does not have EQ Software - You need 3rd party software):
1. Check out specs on your headset and determine if you need to turn the Gain Switch ON or OFF. Check your headset Impedance Rating and Wattage. You may want to consider turning Gain Switch ON if your headset has a high Impedance Rating
2. Obviously, the issue you are having is being caused some frequencies being out of sync
3. Recommend 3rd Party Equalizers:
FooBar2000
APO EQ
J.River Media Player (Not Free, but well worth it)
*There are other you can Research and Review on this website or other forums
SUMMARY
If I were in your position, I would recommend you review the Device Guide, Headphone Specs and visit the FiiO Support webpage (Even though, I don't think you will find a golden answer on support page, you may gain some insight?). If needed, uninstall and reinstall FiiO software, then install EQ software and adjust frequencies accordingly.
P. S. The FooBar2000 EQ is one of the best programs out, in terms of free EQ Software. However, it can be complex to setting it up because it has a vast number of options, plug-ins, tools, etc. So you will need to do your homework.
The other option is the APO EQ, which highly regarded by many audiophiles and somewhat more simplistic in terms of setting up your FiiO and an EQ. Basically, Plug & Play.
If you decide to go with APO EQ option, goto sourceforge.net website and make sure to follow download instructions carefully. The APO EQ involves the download of two (2) programs
IF you have any other questions, please feel free to post
Bobby
PECROBET