Tube preamp (EAR 868) plays music for 10 seconds after being powered off... Why?

If I leave my sources on and power amplifier on after I turn off the pre-amp, music will continue to play for about 10 seconds while gradually fading out. If I turn off the power amp first (Pass XA30.8), the music stops almost instantaneously. 

Why does the EAR 868 do this? Is it common?