recommend an 8ohm preamp for 2x 400w speakers?

Buy a new Crown DriveCore 2 XLS 1502 amp from Sam Ash. $399 (list at $699) + free shipping. More than 300 wpc and 8.6 lbs weight. This new technology amp will sound as good as any multi-kilobuck "audiophile" amp! Truth! Let the rock throwing begin!

The XLS 1502 is scary good and a threat to all the over-hyped and overpriced hi-end amps! 

Drive it directly from your digital source. No preamp needed!

OP:

So you bought professional speakers.... are you setting up a DJ rig, or using them for home? 

Erik 

Mostly, they'll be used with a 3x piece jazz band, at small to medium sized gigs (weddings, cafes, etc). Sound will be coming from a Tapco 220FX mixer, which only has traditional 1/2" jack output. I've not tried yet - should I just get some headphone jack <-> SpeakON adapters, and plug directly in?

Yes!

Your mixer is essentially a preamp!

For what you described, you’ll love the Crown 1502, if you are carrying around Gator gig cases, it’ll slide right in with your other gear