To DAC or not to DAC ?

Wow, that is a hard one. You have two pieces of high end audio gear and a couple of budget components... and looking at another budget component. I guess, I would save up and buy a used DAC... something that would have originally cost well in excess of $5K. Or maybe as a minimum a Schiit Yggdrasil. Then upgrade your preamp to a much higher level. You have really potential here... but I wouldn't recommend tiny upgrades.

Basically your speakers and amp have much greater capability than your other equipment. The real key in getting the most out of your equipment is matching capability of all the pieces. So, you have tremendous opportunity to get better sound quality by upgrading your other pieces.

A Mac preamp is of the right caliber for your system. But make sure you audion it or at least a Mac system before you commit. Many people love it. Mac is a well respected and loved brand, but it has a very distinct sound, heavy on the bass, good midrange bloom but soft in details. It excels at playing rock. It may perfectly match what you want, or not. It would not hurt to listen to a few systems to set your direction. Might try, Audio Research, Conrad Johnson, Sim. 

@headphonedreams

+1 The Absolute Sound and Stereophile Recommended Components issue.

A two box solutions allow greater flexibility to upgrade one aspect... save up for next upgrade... etc. you might save a few dollars with a single box solution... but long-term is unlikely to be as cost effective unless this is your only upgrade.

Cool. Good choice. Take your time, you will be rewarded.