Impossible to have sound quality degredation, based on wireless vs wired - packets are packets, once reassembled, they are identical. Period.
Now, you can have signal coverage problems, I do in the exact setup you mention - PC on 2nd floor, system in basement. I bought a repeater, and have no issues.
Excellent audio piece, a modded SB3. If I can have 100% the same audio as futzing with cd's, and never leave my chair, then call me Jabba the Hutt!