I haven't noticed a significant change in headphone sound with Sense 5. Also to be clear, are we talking wired headset, or Bluetooth?
As to the fix, you've already been instructed (several times) how to downgrade to Sense 4+ in timmaaa's n00b thread; if that is what you want to do.
Alternately, you can try an equalizer app, or flash a custom AOSP ROM with DSP Manager, to see if tweaking helps at all.
Don't see the point in starting 2 new threads, for essentially the same question.
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Phone: AT&T HTC One (M8)
Tablet: Samsung Galaxy Tab Pro 8.4 (WiFi only)