Apologies if I missed the detail on the speakers. Now that you've brought it to my attention, the OP's "lack of bass" and "brightness" observations are fully explained, but the mystery of what the OP is after here is made worse.
B&W 683s are the L & R portions of an HT system, which crosses over at 80 Hz to the ASW610 Subwoofer.
From a review in Ultimate AV Mag.com (
http://www.ultimateavmag.com/content/bw-683-surround-speaker-system-page-2):
"What the B&W sub does do is blend beautifully with the 683s to the point where the overall bass performance simply appeared to be great bass coming from a pair of modest, floor-standing speakers—a result that might embarrass more than a few systems selling for several times the price of the 683s and a single ASW610...
Highs
• Clean, open, transparent sound
• Excellent dynamic capability
• [Sub]Woofer gets the job done while rarely calling attention to itself
Lows
• Can be a little unforgiving of bright-sounding program material, particularly on two-channel music playback"
It wouldn't matter what amp you hooked up to these, they're not full-range speakers, and don't extend into deep bass. They're naturally bright.
As you've already made your purchase from the Classe dealer, having returned the Mc amp, there's little point to continuing this debate.