Lagonda - yes, many people, myself included, bought the Io Eclipse Io (or earlier versions) with volume controls, hoping it would remove the need for a separate preamp. Turns out, it did not (even if it works quite well). I believe this is a typical story in hifi - "can I use my source component with volume direct". Often it turns out, no. The preamp is the heart of the system. - You already have a quite good combination, so this is a good starting point, you can test your Primare 928 SS preamp against others.
Ron - I use the Io volume around three quarters. Not fully up. Depends on tube quality, etc.
You wrote "I used the Io direct to the amplifiers for 18 years. I confirm that Jim White believes the set-up sounds even better with a Callisto between the Io and the amplifiers. Don Saltzman firmly agrees with this, and uses a Callisto between the Io and his high-power tube amplifiers." 18 years! Thats even worse than me
I used 10. The info on this is extremely well hidden. So no wonder!
Yes you will most probably be better off from a Callisto preamp, or, an alternative I have considered, an Atma-sphere MP-1 preamp. This gets great reviews, and I like their OTL approach, I use their MA-1 amps. But again, the MP-1 is a two or three-box solution, and with the Io, I would not spend money on a version with an internal phono stage. There are probably many other good alternatives out there. Surpassing my Einstein The tube mk2. All I can say is, I got it for a reasonable price used, it was a convenient one-box solution, and it clearly out-distanced the other preamps I tried. Plus (I have to admit) it got good grades from Michael Fremer in Stereophile, making him write that the best preamp is the one that sounds like nothing.