I have a little different experience than the comments above.
I have an original Io but I had it updated by Steve Huntley of GNSC with the same Dynamicaps as the Io Sig. Many other parts have been replaced beyond the Sig, mainly Caddock resistors throughout. I have considered changing the other caps in the Io beyond the stock RelCap parts but have held off with this. Steve also noted that just from a quick glance inside the power supply, parts could be changed to bring on significant refinements as well. And now that he is an Aesthetix dealer, he has a lot of knowledge of this product line.
The above changes did not affect the original Io's tonality but simply brought on a stronger bass foundation and significantly greater dynamic contrasts. The incredibly rich and textured midrange, a strength of the Io, was thankfully retained but now more clear. There was a little more openness on the top and I suspect it had more to do with a refinement of treble clarity rather than extension. The overall presentation of the Io's top-most coverage was/is still a bit soft and clearly attenuated.
I can not imagine running an Io or Callisto with stock tubes. There is so much benefit here to get those horrible Sovtek tubes out. People spend much time playing around with various cartridges, turntable leveling, isolation, VTA, etc. To not put the same effort into trying premium tubes for a product of this caliber just makes no sense. And a "mediocre" tonearm cable can so quickly destroy much of the potential here as well.
For the longest time, I had a noticeable tube rush with the volume high and the music material at quiet passages. I had been in contact with other Io owners (Sig and non-Sig) who had the same tube-rush issues. There was a problem with early units with the power resistors. These were later changed to Roederstein for all subsequent products. When I changed these in my Io with a free resistor kit from Aesthetix, the noise level was much the same so the noise problem was elsewhere. I changed the resistor values in the RIAA circuit with an update kit from Aesthetix as well.
Ultimately, Steve at GNSC determined that a tube-socket in one channel was noisy and the pair of input tubes on the other channel were noisy. Both of these finally resulted for me to get the Io to a noise level I had never experienced before. Clearly the older parts, noisy tubes and a noisy socket all contributed to the noise issue that relates to what other people have reported about the Io. But I suspect that the vast majority of reported noise issues with the Io is because someone has not gone through the effort to find a quad of ultra-quiet tubes for the input stage.
Telefunken 12AX7 tubes are wonderful here, but it takes time to find 2 pairs for the CRITICAL input stage. The Io takes 12 12AX7 tubes and the Callisto 2 more so I bought 20. I went through all I had to find 4 for the Io's input stage. So many of these tubes worked well in the other stages but simply were too noisy for the first stage. Ultimately I found myself running with 10 Tele 12AX7 and 2 Brimar 12ax7 on the final stage to bring on a little more presence in the trebles. It was a trade-off between the midrange magic of the Tele and the more tonally-coherent Brimar.
It was long after I learned much from the 12AX7 effort that I also learned that the 6922 tubes here were as critical. I tried many here, Mullard, Siemens, Amperex but the Tele 6DJ8 was outstanding. I was really floored by the added smoothness and reduction of grain in the mids by just the 6DJ8 tube over the Sovtek 6922. I played with the 6SN7 but the other tube here that truly makes a major improvement is the pair of EL34's in the PS. I tried so many EL34s but the old Mullard XF2 along with the much less expensive JJ bring on yet another level of benefit. All the tube changes here were far more significant than the parts changes to take this Io to beyond Signature status.
If the Io has no volume controls, it needs to drive a line stage: the Io MUST be used with XLR cables to achieve its magic. No other product that I have tried was so responsive to XLR vs. RCA. And I used the exact same cable brand/model for this observation. The difference here was not in the way that many people write about the benefits of balanced connections. The RCA connection here simply robs so much of the dimensionality and decays that are the Io strengths. My CD playback system into the Callisto Sig did not exhibit this same difference between XLR and RCA cabling so I concluded it was more the Io than the Callisto that excelled with, or was sensitive to the XLR connection. So keep this in mind when you connect the Io (without volume controls) to your line stage.
My view on the Io's resolution differs from Rushton. Perhaps 5 or so years ago, I might have considered the Io to have exceptional resolving capability but that is just not the case today. Direct comparisons with the new Aria WV show that the WV brings on so much information that is suppressed by the Io. And maybe the newer "updated" versions of the Io and Callisto Signature models address this primary weakness of these otherwise outstanding preamp models.
John
