Hey....the Alpine knob looks fantastic on the Rio. I think I will have to follow suit and change mine as well...and I too have the Alpine CDA7878.
Funny you mention the 'hiss' you hear when in AUX on the Alpine. I have mine hooked up the same as you and I too have the same background hiss. I seem to get the best results though by turning down the volume on the Empeg to about -6db or so. Seems to sound cleaner as well like this...