Looking at the AOSP repository, 4.3 still has the default a2dp bitrate at 229, which is horrendous. 4.4.2 is supposed to have it set to 328, so I'd want to test if a newer file will work on 4.3 or not. The chance is slim, but so far nothing else worked, and I don't want to update to 4.4.2 and lose root.
So could someone who already updated please upload the 4.4.2 version of the audio.a2dp.default.so file? It's in /system/lib/hw/ and you don't need root to copy it (as far as I know).