dac2 - slightly better low level resolution with the Burr Brown chips
dac3 the tube stage gives it a warmth that can sound too warm in some settings - it is a very engaging piece
most prefer dac3 to dac2, but there is a price jump
both are built like a tank
nos tubes telefunken, amperex, etc
talk to fletch here on audiogon
he can match tubes for system characteristics with the best of them
both daca mate well with arc preamps
you may also want to research the newer dacs with upsampling etc
hope that helps
Tom