@thoft, the Adcom GFA - 5800 is rated as 400 Watts into 4 Ohms:
https://www.manualslib.com/manual/206141/Adcom-Gfa-5800.html?page=11#manual
While it might very well be up to the task, one can see that it's not quite up to doubling down from 8 Ohms to 4 Ohms and there isn't a 2 Ohm rating. Unlike your 3.5's these model Thiel's have sub 3 Ohm loads:
https://www.stereophile.com/content/thiel-cs37-loudspeaker-measurements
https://www.stereophile.com/content/thiel-cs72-loudspeaker-measurements
https://www.manualslib.com/manual/206141/Adcom-Gfa-5800.html?page=11#manual
While it might very well be up to the task, one can see that it's not quite up to doubling down from 8 Ohms to 4 Ohms and there isn't a 2 Ohm rating. Unlike your 3.5's these model Thiel's have sub 3 Ohm loads:
https://www.stereophile.com/content/thiel-cs37-loudspeaker-measurements
https://www.stereophile.com/content/thiel-cs72-loudspeaker-measurements