Using the headphone jack output on an iMac into the analog inputs of a preamp isn't going to sound very good, as you've discovered. Doing that means you're getting a signal that has already passed through the low-quality digital to analog converter and amplifier inside the iMac.
The simplest way to improve the quality is to get a decent digital to analog converter (DAC) with an optical/Toslink input. Connect that input to the iMac headphone jack with an optical cable with a Toslink connector at the DAC end and a mini-Toslink connector at the iMac end. Then connect the analog outputs of the DAC to one of the analog inputs of your Outlaw preamp.
The simplest way to improve the quality is to get a decent digital to analog converter (DAC) with an optical/Toslink input. Connect that input to the iMac headphone jack with an optical cable with a Toslink connector at the DAC end and a mini-Toslink connector at the iMac end. Then connect the analog outputs of the DAC to one of the analog inputs of your Outlaw preamp.