Well, it is a tough call. The good thing for you at this point, is that not only is the Rega a decent performer, but it also holds its value pretty well. I'm quite sure that if after a year or so the upgrade bug does bit you can always sell the rega for that same $350 and not be out anything! Hell, that being said, buy it now and wait for a TubeDac in the future - when one finally does show up (from my experience one will go up for sale as soon as you committ to the Rega) you'll have the best of both worlds, better tunes while you wait, the opportunity to A/B the Rega and Nixon, and won't be out any more money in fact you may make a couple bucks on the Rega.
I've owned the CIA and the Nixon but have not heard the Ack. I compared the CIA directly to the Nixon and the Nixon to the Mensa DIO. I MUCH prefer the Nixon. Others have high praise for the Ack, especially since it's battery operated. I've really got no wish to change DACs. I like the Nixon that much. It's really a wonderful piece. When I listened to the older Rega I found it to be less musical and revealing that I had hoped.