I've been using my 3.5's with eq on full boost in a moderately large room (15' ceiling peak) and play music at the volumes which were intended, scaled back for domestic room size, in other words; fairly loud on peaks, with an amp capable of 500 Watts (and double that for a couple of minutes at a time) per channel into their 4 Ohm load for many years now, without a hint of problems. Though, I do wish they could play even louder on dynamic peaks with less strain. My 3.5 midranges drivers were rebuilt by Thiel shortly after I purchased them. The previous owner used what IMHO was an underpowered receiver to drive them. Again, no problems since.
Not using the eq would be an anathema for me. Even on steadily loud rock music, I would use the eq at least on the 40 Hz setting. With more lowered tuned 5 string basses and synthesizers being used, there's too much music below 70 Hz where the eq kicks in to compromise without it . That coherent sealed box deep bass is part of the reason I like the 3.5's so much. Without it, I might have kept my old CS 2's. I still prefer the 3.5's to the later Thiel offerings, except of course for the only other sealed box Thiel's; the CS 5i's. The amplifier requirements for which are the only obstacle towards my ownership.
That up to 12 dB eq boost might seem a bit extreme, but it is mitigated somewhat but the concurrent rise in impedance.