At first I'm gonna try to shortly point out the differences between the three versions of the ECHO INDIGO:
Indigo (standard version, not produced anymore): has two headphone outputs which can only be used with one and the same signal for both outputs (and the same volume setting)
Indigo DJ: has two outputs which can be used independently (one line-out and one headphone output)
Indigo IO: has one headphone output and one input
But what does this mean for their sound quality?
It's sometimes said that the line out of the Indigo DJ is superior to the headphone out on the same card and to the headphone outputs on the standard Indigo and on the Indigo IO.
I doubt this because the DJ uses the same DAC and quite likely almost the same circuit to output its line-signal that the two others use on their headphone out too. And this line-signal is somewhat amped as well, just as all line-outs are. The only difference that I see is that on the line-out the potentiometer is bypassed whereas the signal on the other cards can be attenuated. In fact I'm using my Indigo's (standard version) headphone out as a line-out to my speaker system. I just turn the potentiometer so that I get the same loudness that my CDP delivers and I can't imagine that the sound is worse than that from the Indigo DJ's line-out. This might be an interesting question for everyone who's unsure whether to go for the DJ, IO or (used) standard Indigo. As I'm not an expert I'ld appreciate comments and explanations on this topic.