Ok.. Great!
Before replacing your fx audio x6, may I make 2 suggestions?
Here is some info on the msr7/b you may not have read:
https://diyaudioheaven.wordpress.com/headphones/measurements/audio-technica/ath-msr7/
It appears there are fakes sold. Even if it is genuine, it just might not be to your taste. So, can you audition some other headphones using your current source and the fx audio? Perhaps see if you can listen to an open back, such as a HiFiman HE400i or similar?
Generally, in the string of components involved in headphone systems, the mechanical transducers', the headphones, will be 'the weak link'. Or perhaps better said - the device with the most possible 'deviations'. The DAC is usually one of the components having the least 'deviations'. You will find members here arguing that a $9 Apple dongle can be more accurate than a certain $5,000 DAC. Likewise, unless you are listening at ear-splitting volume, the Amp portion, unless it is unusually poor, is also usually not a severely limiting factor.
Next, try changing the source. Perhaps from the PC to your phones 3.5mm audio jack, or with an OTG cable, the Fx audio USB in. Or from your TV toslink out. Try different music sources - perhaps a 'lossless' one on a free trial.
Lastly, try plugging your msr7 into some other headphones source, your AVR, your stereo, your phone, even your TV 3.5mm. If it sounds better, you may actually have a defective fx audio.
I mention all this as I am thinking a change from a $65 DAC/Amp to $150 DAC/Amp may not bring you much..