I just finished auditioning both of these, and the punchline is, the Benchmark went back for a refund and the ACK! is staying!
The Benchmark is a great DAC, no doubt - I can see where it garners rave reviews. It is highly detailed, very transparent, very exciting listening. But after awhile, its relentlessly detailed presentation gives you the proverbial digital headache. I started listening less & less. It would be great for studio use though.
The ACK! on the other hand reminds me more of my trusty VPI turntable and records - everything's there, and everything just sounds right, just like music should. No glare, sweet but not sugary, nicely balanced, dead quiet. And, you can listen for hours on end - you have to drag yourself out of the listening room because it's time to go to work. Bonus - it's half the price! Worth the wait.
In my modest system - Quicksilver monos, B&W CDM NTs, McCormack passive pre - it was no contest at all. Good luck & good listening!
p.s. If you love good female vocals, check out Christine Kane's latest CD at www.christinekane.com Fantastic!