need help selecting the right preamp

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 above post is good. Another alternative is to get a good USB to SPDIF converter (Audiophilleo 2 is good) and a DAC with S/PDIF in. Some knowledgeable people claim the clock in the USB/SPDIF converter has more impact on sound than the DAC itself, but you will still need a DAC.

Yet another alternative is getting a DAC with asynchronous USB input, and go from iMac to DAC without the above-mentioned converter. Some examples: wavelength, Wyred4Sound, Ayre, Empirical Audio.

Thanks for your input. I found a good solution after speaking with Apple tech support. The headphone jack line out from the imac actually doubles as a fiberoptic output. Toslink makes an optical cable with an adapter to make it fit into that headphone jack. I was able to hook up the other end directly into the optical input on the preamp. Good music :)