I had a M2si here last summer, I thought it was a sweet little integrated at that price. At the time I was looking for a relatively low powered solid state amp to replace tube amps for summer listening, looking for low heat output. I generally used it only as amp, bypassing with my Coincident Statement linestage powering Klipschorns. Also used it full function powering Merlin VSM-MM, about same 91-92db efficiency as your Triangles, no problem driving those.
I'd expect M2si to be sweet match with your Triangles. To reiterate, I was very surprised by this integrated, I'm used to running $5-10k tube amps with my Klipschorns, I didn't feel deprived running M2si. Ultimately, I did sell to a guy running Maggies, very difficult load. I mentioned possible issues with this, and asked him to notify me if he had any difficulties. I assume he was ok, never heard back. He did mention he only listened at moderate volumes. I'd also add I modify much equipment, very impressed with build quality, especially at $1k price.