Just to throw a little gas on the fire (actually this is a serious suggestion, but may not be well received by all), Ryan did say he was looking to match the 481/483 sound and not the label, so I wouldn't rule out other brands IF they sound like 481/483s.
Some folks describe the Audix Micros (M1280 or M1290) as sounding a lot like the 480s. I'd suggest searching the archives on this site to read about folks making the comparison, and more to the point listen to recordings made with the 480s, 390s, and Audixes. the Audix are small enough to just about pass for "actives" as is, and there's the Church mod for plug in power if you want tiny. With patience, a used pair with cards and hypers and mounts and cables should be well within the $600 budget. Now whether an Audix M1280 or an AKG 390 sounds more like an AKG 480, I leave that to Ryan's ears.