I just upped my HU from the RF RFX9000 with 2.3v pre-outs to the RF RFX9210 with 5v pre-outs and it made quite a difference. Its not that the sound is much louder, but its much more full, and much crisper. Its really about clarity, the amps are going to make it loud regaudless, but if the signal is distorted, the ampification will also be distorted.

The system sounds 100% better with stronger preouts. (IMO)