GOT IT! (And it was self-inflicted and I am a moron)

I plugged my kid's keyboard in and found the same lag, which eliminated the KS88 as the problem source.

Then I routed the sound through the monitor via the Onboard AMD sound processor and disabled the Sound Blaster.

Boom. Lag gone.

Sound Blaster does all this processing for games and movies (though I think the sync issues would drive me nuts there, too) so I just turned off all the special features and the lag went away.

Thanks for the walk-through, guys. That really helped me narrow it down.