If you are fine with your Xonar D1/DX DAC then OK, but OP meant external DAC for playback only and in this case you can't compare 100USD sound cards with 1000+USD DACs. I started with DAC19MK3 (500USD) and it was mind blowing experience (as I wrote: bigger soundstage, better transparency, better 3D imaging, more details, better separation, analog sounding, and so on ) then after three years of happy listening I've moved to Reference 10.32 (1850USD) lately and it is thrilling to listen to it - it blown DAC19MK3 out of the water - I have never had a feeling being so close to the music before. This is not to say that ridiculously overpriced Hi-End audio gear doesn't exist, but you can still buy reasonably priced DACs and hear the difference.
If you are really sure you hear a large difference, why not take the test I suggested ? After all, if the sound card DAC is degrading the quality, it should be audible on its recorded sound, too, unless the recording somehow magically undoes the damage (in reality, it most likely just causes even more degradation, therefore making the difference appear greater). In subjective evaluation of audio gear, if the comparison is not done under well controlled conditions, it is hard to know when one really hears a difference and when it is imaginary, under the influence of price tags, aesthetics, and other non-audio factors. You may feel sure you really hear an improvement between your DACs, but so do those who buy the "high end audio gear" you consider ridiculously overpriced, $10000 power cords, etc. Perhaps in reality a $1850 DAC is already in that category ? There is technically no reason why a $100 sound card DAC cannot be good enough that it does not audibly affect the sound stage, imaging, details, or whatever else.