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

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

Your mixer is essentially a preamp!

Yes!

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?

OP:

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

Erik 

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

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

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!