Blinxat
500+ Head-Fier
Interesting that you say that, that because I've seen a very serious audio review insisting the 816 sounds nothing like the M8.
http://www.gsmarena.com/htc_desire_816-review-1079p7.php
http://www.gsmarena.com/htc_one_m8-review-1062p7.php
They say similar things about it and the measurements are nearly identical.
Thats how I heard it. I can't AB because I gave the 816 away but when I got the M8 it felt like getting the 816 back. Despite measuring flat they both are a touch warmer and more punchy than the Nexus 5 I had in between.
The 820 however uses a different chipset and posted poor numbers. But the 816 was great.