Before you buy on baseless reports, find out if everyone has heard the player they lament or praise. Personally, from the list above, I will have to say that the Sony is an excellent player which has the best bas resolution of any of the above. I will not comment on its D-amp as that is marketing speak of Sony and Sharp. The S9 is decent, but is bettered by the D2 which can use Rockbox and a real EQ. Both cowon suffer fairly serious bass roll off, but rockbox is hard to beat.
I have not heard the Teclast.
The Nano 5G is in some ways, similar to the Sony. Both have pretty weak headphone outputs, very similar distortion and stereo images, but the Nano has a more typical Apple nano sound: reedy. The Sony is thicker and elevated in the bass, but moving away from portable earphones, loses power and punch.
All of the above (except for the teclast which I have not heard) are great choices. The teclast may also be, but I would not stake that claim on the rants here.