> PC soundcard outputting the original source would be better.

Said that already.
Don't be boring.

> I can only imagine what 2nd generation ISD sound is like

That's the way the human brain works.
But quite likely it would be reasonably OK.
I used an ISD2590 in a disability aid and therapists using it said 
that the speech quality was better than almost any other product in 
the field. The ISD2560 should be a substantial improvement over that. 
So a double pass may well be quite OK. I can try that easily enough 
using speaker to mic transmission with two of my devices. Using 
properly matched electrical connections should be substantially 
better.




        RM

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