Differences in sound CAL Icon, Sigma, Alpha ?

Can't speak to the Icon, but I have both the Sigma and the Alpha, which are very close tonally, so close that I doubt your could tell them amart on casual listening. However the Alpha is better resolved, detail wise, and does a bit better at imaging. I also like the Alpha also because it has multiple selectible inputs and it has a sleep function - you don't have to go behind the DAC to turn it off, and its fast coming out of sleep when you turn it on, otherwise it takes about a half hour to warm up. The Sigma has a bit of transformer hum - the Alpha is dead quiet. At the prices I see on Audiogon I think the Alpha is a much better deal.

Ditto on the above, I compared the Icon on its own and with both the Sigma & Alpha. The dacs add much more stage size, better resolution and bass, in addition to flexibility (tube rolling & digital cable interface). Ended up with the Alpha & Mullard tubes. Sounds better with the top off too. Aftermarket cords take it up farther, still a great deal at the going prices.

I had an Icon II for several years, until it started skipping, then moved to a Delta transport/Alpha DAC combo. When I had the Icon, I usually coupled it with a Sigma II DAC and liked that sound better than the Icon alone--primarily, the Sigma took the "edge" off of the digital sound more so than the Icon by itself. Right after the Icon died, I used the Sigma to warm up the sound coming off my Philips CD recorder , which otherwise is pretty bad as a playback unit. That combo wasn't as nice sounding as the Icon/Sigma pairing though, thus my purchase of the Delta/Alpha for serious listening. Now, we're talking!

I'm using black plate Sylvania 5751 tubes in the Alpha and sound is marvelous--lots of air around the instruments, palpable glow around vocals, cymbals and vibes sound as if they are right next to you, and deep nicely resolved bass. I liked what I was hearing so much that I retired my solid state amp and preamp and got a integrated Jolida tube amp, which was another good investment. Wish I'd done that years ago.