Need integrated amp with built-in USB DAC

A used Naim UnitiQute appears to meet all your requirements. Other units to consider would be the PS Audio Sprout, and the Sonos Amp. Neither the Sprout or Sonos are "killer" pieces, but both will attach to a Mac. The Naim has much better sound, but will cost about 2 to 3 times more.

Also, count me as a +1 regarding a used Ax-7e. I have one of these and concur with the comments from others. The Ax-7e is a great value, especially if you can find a used one for sale in clean shape. Hope that helps.

Purchase a Behringer UCA202 to add optical out via USB to the Mac. Then you can get a unit with a toslink optical connection. You'll have way more options going that route than restricting to ONLY units that have USB.

http://www.amazon.com/Behringer-UCA202-Audio-Interface/dp/B000KW2YEI

This may be perfect for you. Price is even a bit lower than your limit, but it is worth more.  It is still a pretty new model.  Its from the same people who gave us the high-end Nuforce products ... NuPrime IDA-8

There is an expired add in this forum.. Look here:   http://tinyurl.com/o8mhjcj
I want to get one myself.

Gene

Dared i30.Peachtree Audio.