Hi Wes,
Over here B&W use Classe for their dems, no doubt due to them having European distribution.
A used CA101 or CA201 should be in your price range, although compatibility with the Showcase would be an unknown. Still for surround duties, their qualities should be ample. An alternative would be used Krells, maybe KAV250 or KAV150. Also there are many fans of BelCanto over here, a used Evo 200.2 is within budget.
As Julian says, realistically, I find it preferable to use matching amplification on all channels, the last thing you want is the rear soundstage to somehow sound different.