Are line-out jacks needed to use an ADC?

That will most likely work fine.

The only possible problem I can foresee is that if while you are digitizing via the adc you are also listening via the normal signal path (preamp to power amp to speakers), the volume control setting on the preamp that you prefer for listening might not be as optimal as possible with respect to signal-to-noise ratio at the adc input.

That would be most likely to be an issue if your usual volume control setting for listening is near the low end of its range, say around the 9 or 10 o'clock positions. To the extent that noise pickup in the cabling to the adc may be significant, or that noise that may be generated in the adc's front-end circuitry (ahead of its gain control) may be significant, supplying a higher level signal from the preamp (corresponding to a higher setting of its volume control) would reduce the level of background hiss in your digitized files.

But very conceivably that won't be an issue at all.

Regards,
-- Al

Thanks very much, Al. You answered my last question, too, which I also much appreciated.
-- Howard

If I understand correctly, the 'passthru' function of my integrated will put out a unity gain signal.
This should be maybe? 2v, which would be fine for you ADC.
Can't listen at the same time, though....unless you want it real LOUD.