The E10 is a definite step up over the built in sound card on your Imac. I either own or have tried extensively all the products you have listed there....besides the Schiit.
The A2 is more thick in the mids. Some love it. I personally find it to sound too hi-fi. I like my music presentation neutral yet engaging, without too much focus on mids or vocals.
The E10 is excellent for the price. Although nothing special compared to high end stuff, even to my Marantz CD player. Excellent way to start out though. I did the same.
The HD598 are quite fun to listen to, although lack the bass for electronic or even hip hop. I personally went with D2000 over them initially when making my decision.
The D1 should be better than E10, although until you hit that $400+ mark, almost all the products are limited to the same performance. They'll have different sound sigs, but neither will have that polish though.
LOL I hope this doesn't sound demoralizing. Take it as a solution almost. I'd try to pick up maybe a E10 or some cheap used DAC. Don't skimp out on DAC. You want to feed the speakers and headphones as well as possible. At this price, maybe go for separates so you can mix and match later on.
Maybe look at other powered monitor based speakers around that price, depending on your music preference.