Call it the Big Box Blues, a nasty combination of lowered consumer spending, high rents and aggressive online competition. The blues hit Best Buy’s president today as he resigned abruptly as boss of the huge electronics retailer. Best Buy head office announced that Chief Executive Brian Dunn (left) resigned as part of a top to bottom re-alignment of the firm. The company faces stiff competition from discounters and online retailers. It’s missed sales expectations for several quarters in a row. Best Buy said director Mike Mikan (right) will serve as interim CEO while it looks for a permanent replacement for Dunn.