It's unfortunate the Netherland company is not well know because it's a great amplifier.
I have used it with my Fortes and it was an exceptional match in a larger room, but it also sounds very nice w/ my lower efficient Seas towers. It drives all my speakers, including my Dynaudios, quite well.
The amp is quick and clear with a modern sound for a tube amplifier, probably because of being biased in Pentode mode and using 6sn7 preamp tubes.
I have an acquaintance who recently purchased a preowned IA-21 version and loves it w/ his vintage ADS...maybe it was yours he purchased.
I had the Mystere IA21 but sold it due to a speaker change ( needed more power). Now I have a pair of 101db speakers and totally regret selling the Mystere. I think me and you are the only ones who have had Mystere in our systems throughout the years here in the Virtual Systems. Did you use the 11 on the Forte’s? I bet that would be good synergy. Nice system as always!