The PDX amps have won IASCA SQ finals so it's not the S/N ratio. Perhaps you have a bad amp, but highly unlikely. To make sure the noise is coming from your HU rather than from inside the amp, you can disconnect the RCA's while the HU is on and then listen for noise or hiss. If there is nothing, then your amp is fine.
Like Technic pointed out, a line driver may be a good choice to raise the S/N ratio of the head unit. Or you can re-connect the factory amp which is virtually the same as a line driver. The factory amp however is "optimized" for your HU and may give you the best results.