Wouldn't have believed it if I didn't experience it myself:
Just bought Audioengine A2+ powered desktop speakers. All the reviews, Stereophile, TAS, blah blah - had high hopes going in. Wired them up to a Win7-64 PC, standard connector to line-out from RealTek HD Audio controller (no USB). Fired up variety of .flac from XBMC - and mid's and high's from the A2+'s sounded like crap: poor extension, muddy at low volumes, narrow & flat sound stage with too much bass.
2nd-guessed the sh!t out of it: connections, sources, audio controller. Looked for posts on EQ, suboptimal Audioengine factory settings , speaker placement. Then I found this, in a head-fi.org post from 2010:
"In regards to the Audioengine A2, try reversing the insertion-direction of the short A/C cord which plugs into the A/C adapter. I found that inserting it one way versus another will produce a better soundstage and sound quality. Give it a try and see how it works for you."
That was with the older A2's but what the həll, I flipped over the AC connector into the A2+ power brick...
...and that fixed it: mids and highs approaching what they should, and a soundstage worth the purchase price. Can probably get it better with EQ at the source, but bulk of the shortfall was addressed.
Who knew...am still incredulous. No mention of this in the A2+ setup guide or any other Audioengine docs afaict.
Interesting - thankfully the plug into my PSB Alpha PS1's only seems to go one way, and that's in the general direction of awesome :D