re: flat sound
I have quite a few earphones & their sounds all vary when using it on the D810. If you like a bit of bass & good mid-range
I recommend the Creative CP630. I suspect Seinnheisers CX300 would be the same.
I too found adjusting a2dp with Fit4cat made a difference in sound quality.
I bought the Sony MDR-EX90 recently that sound fantastic on my home stereo, but sounded very flat on the D810.
@ Ipnotik : worth trying out a different pair of earphones. Dopods may not be the best mp3 players, but hopefully you get to find a good headphone combo which sounds bearable.
Oh & try Mortplayer as it has an equalizer .