Bro you might want to check the panning laws of your DAW when it masters the track.
I had this problem before na when I bounce my track sa ADOBE Audition, may volume drop na malala
so ayun, I googled it and apparently apnning laws ang may problem.
Also, baka naman may special EQ settings ang soundcard mo? kaya pag sa audio interface mo, maganda ang labas pero pag pineplay mo na sa soundcloud via your soundcard, iba na ang tunog?