In my setup I had done some change since 5 months, but after so many months I realize I am not enjoying it. 
I am not able to find what is causing this. I will try to explain.
Original setup: [sounding good]
apple ipad via wifi --> apple airport express having Airplay --> SPDIF ---> DAC
Revised setup : [more clarity, less warmth]
apple ipad via wifi --> Raspberry PI having Volumio with Airplay --> USB Out to Topping D10 , being used only as USB to SPDIF convertor ---> DAC (same as above)
Would volumio might be causing it, or the Topping D10.

I am not able to find what is causing this. I will try to explain.
Original setup: [sounding good]
apple ipad via wifi --> apple airport express having Airplay --> SPDIF ---> DAC
Revised setup : [more clarity, less warmth]
apple ipad via wifi --> Raspberry PI having Volumio with Airplay --> USB Out to Topping D10 , being used only as USB to SPDIF convertor ---> DAC (same as above)
Would volumio might be causing it, or the Topping D10.