It depends. 8-)
Back in the olden days with those high falutin' tube amps, the "ideal" was to have a pre-amp where the output impedence was about 10% of the input impedence of the amp.
It is possible, with a large enough impedence mismatch, and a long run (or poor quality interconnect) to create a situation where the signal could be attenuated a few dB.
I'm too lazy to look up the output impedence of the 990. But if it's 1K, then any amp with an input impedence over 10K should be fine and result in little measurable signal attenuation. In this case, I'd say it's not an issue (by a rather wide margin) and I'd definitely choose the 990 over the 970...hands down.
Cheers,
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