I have noticed this as well...the CD's played through my second head unit ALWAYS sound louder...and richer...than when using the Rio as the source....with the sony i can only turn it up half volume..with the Rio almost full volume...and i have the Sony fed into the AUX in on the Rio

....not for nothing...but CD's sound sooooooooo much better than MP3's at 128 kps..or even WMA at 160 kps