I've now discussed various options with Glenn and Jim Aesthetix - a big plus for customer service, even for second-hand owners - thanks. And with others.
What it boils down to is this. I write it here since it may be of interest to other purist analog system owners also.
Even if a line out can be made from the Io, it needs to be buffered, and another buffer will degrade the sound, Glenn writes, having spoken with Jim.
I note, "another" buffer. The potmeters on the Io is among the important changes in the Signature edition (that I don't have). The most purist sound might perhaps be had by avoiding the potmeter altogether, using a cable with a fixed level/resistance. I have seen Krell fpb600 amp owners write about successful experiments, dropping the comfort of being able to change volume. One owner wrote of having two sets of cables modified to two levels, comfortable easy listening + audiophile. The moral is "direct from source", "straight wire" - though one has to find a balance point with convenience.
Possibly the least sound degradation would be with a Y cable from preamp out to (1) the recorder (pc, dat) and (2) the amp, using a passive volume control on (2) to get high enough signal to the recorder without getting too much volume on the system.
Even this runs into trouble, however, since plugging into the balanced run from the preamp to the amp (a Y cable to some non-balanced recorder) would probably impact negatively on the sound (the balanced operation makes a big difference with the Krell, and I guess, most good amps).
I accept this - I bought into a purist concept. So what I will go for is the "room listening" strategy, not the "line out" strategy. I accept turning off my system before making recordings. But I invest in not-so-expensive computer speakers that can be driven from the recorder's line or headphone out.
Now, I should be able to record from my lovely vinyl recordings to data media without the hassle of having to stand by looking at the spinning LP (to pause recorder before the end or the bad track, etc) or sit nearby using earphones. It won't be great listening, but it will make it easier to record while doing other things.
If others are interested, I'' report back how this functions.
Do you have suggestions for a good data recorder? My Sony Dat records 20 bit, and it makes a BIG difference, better than CD (20 x 48). The best SACD recording beat it, as expected, although they are not as good as the vinyl resolution.
A laptop card would be convenient in my case, but perhaps a standalone box with a hard disk is better. The main point it should give my great analog output (30 bits x 400 something?) the best possible data treatment. The biggest data file, in practical terms.