Fullrun,,
Another factor to keep in mind is the server computer. My PC is highly optimized and I did notice a significant sound improvement with certain upgrades with the Audiophilleo+Metrum, like using Windows Server 2012 with AudioPhil's Optimizer (that's a software upgrade). My point is certain DACs are more sensitive than others to what they are being fed even if all of them have asynch USB. My Audiophilleo has asynch USB and has the PurePower option and you would think it would be insensitive to the computer feeding it, but it's not - and it's a very good converter!
Your MacBook Pro, just like any other laptop, won't be a top notch server. Sorry to be brutally honest here - I do not mean this in a mean way. But a laptop cannot take hardware upgrades like desktops can. I suggest you take this factor into consideration when selecting your next DAC. Are you willing to spend on a top notch server? If not really, then place extra value on those DACs that really are designed to isolate their performance from the quality of the server feeding them.
I say this because isolation from the server quality is a design goal for exaSound. So much so that exaSound president was giving me a hard time because I was driving the e22 with an optimized server running WS2012.
BTW, look also into Lampizator. They are VERY well liked and you can order one tailor-made to your needs, including a very good volume control. If my end goal wasn't multichannel I would be looking VERY seriously into Lampi.
Auralic Vega is very well liked by many computer audiophiles too, and I believe it has a good volume control too. At that price range there are many very good units with great volume controls. Have you checked computeraudiophile.com?