I started with the Io 13 years ago. It had the volume control which is ideal for my setup. Several years later, I upgraded it to the MkII and added the second power supply. Several years after that I upgraded to the Signature version. About 1 1/2 years ago, I did the Eclipse upgrade. Each and every step along the way, the Io got better. Better in terms of lower noise floor, better dynamics, better transparency, more detail, yet more flow and music.
If in doubt, start with a used Io and one power supply. If you like it, they are fully upgradable. As several have pointed out, the Io is very reliable, easy on tubes and the factory support is superb.
Tweaks I made along the way: switched the EL-34s in the power supply to KT-66, replaced the umbilical cables with higher grade wire and lots of shielding. Replaced the stock AC cables with Nordost cables.
The Io sounds best with XLR outputs, much better to my ear than the RCA outputs. If you can run your amps with balanced inputs, that's the way to go.