There are several comparisons online where most conclude that there is little to no difference. As far as sample rates, I read the two formats should convert to about the same rates. I honestly am new to the whole digital thing myself, and because of how little I know, I decided to try to play files in their native format, just to avoid these exact issues.
If you're using your computer to control volume, you're gonna get way more loss in sound quality doing that than converting files. You'd be better off having an external component control volume. The LA-QXD can control the volume with less loss in SQ than your computer.