I have recently bought the SWAN D-10080iv and using it with FiiO 10K. The sound quality is excellent and the you will definitely not miss the bass. My room size is 12x10 and it these perform better than my expectation.
Also regarding the low volume. My setting are, windows volume at max, bass and treble on the speaker at 0 and speaker volume knob at 50. for music or video I adjust the volume through the specific application. Better would have been if you have the application volume at max and adjust the speaker volume but I am lazy
. Also using wireless keyboard and mice from a distance of 10ft. So u can guess.
Coming back to the point. Did you try a different cable 3.5mm to RCA........I am not using the one that came with the package, instead had a belkin 3.5mm to RCA gold plated(don't know if it matters) lying around, so using it. But initially test it with the provided cable....had no problem but the belkin one seems to give a better output. Just my opinion.
And use a DAC to justify these speakers.