For those getting night/day type dramatic differences I'd recommend a re-load, with a clean SD card... take the time to do it right, start from scratch! Personally for me trying to distinguish the SQ difference between 7.1.2, 7.1.3 and 8.1.0 is akin to picking fly poop out of black pepper. There "may" be a difference but its insignificant to me and not worth my bothering. Save for the enhanced feature-set of each subsequent upgrade I could listen to any 7.X-8.1 FW SQ-wise and be happy for life. The only temporary SQ setback I suffered with 8.1 is probably (Thanks J-M!) the SAM V1-to-V2 upgrade which several have commented on and resulted in me and them backing off the bass setting by a factor of just at or over 50%. Other people's SAM setting experience remained unchanged with the upgrade and still others eliminated SAM in 8.1 and are quite happy. Beyond SAM or a corrupt 8.1 FW upgrade I struggle with finding any reason not mentioned above for a dramatically bad 8.1 SQ upgrade experience.
Statements in my posts are opinion only, not to be construed as fact. Any projects I engage in are at my own risk! Their outcome cannot be assured and may result in success, small/no change or catastrophic failure. I encourage no one rely on anything I say or do as gospel and to realize your mileage may vary!