thoft - let's consider possibilities. Your 18'x 25' room with a non-flat ceiling sounds pretty ideal to me for a pair of 3.6s. The Thiel room where duramax heard the 7.2s probably had the Krell FPB 600. That room is 14'x 22'x 35' for 10,780 cubic feet. Your 18'x 25' room has significantly less volume, depending on the particulars of the vaulted ceiling. As stated, a major design attribute of bigger speakers is to provide more bass into larger rooms. However, the 3.6 bass fills the Thiel listening room very well. I would never call it bass-shy, much less 'extremely weak'. Your room is significantly smaller.
I imagine you explored speaker and listener placement to minimize room modes.
I don't know whether your Classe 25 amp delivers current into a 2ohm load. I read reports of it delivering to 2 ohms, but its specifications rate to 4 ohms (minimum). Your amp running out of current at Thiel's 3 ohm load is under suspicion.
The 7.2 and 3.6 are in the same league for impedance brutality, rarely climbing to 4 ohms.
Another thought is something being wrong with the speakers. Your symptoms could be caused by one woofer being disabled in some way. Do you have any test equipment to compare the 2 speakers? Tone generator, REQ, etc. If not a momentary connection to a 6 volt (9 volt with greater care) battery can show the polarity and vigor of the woofer response. Listen as well as watch what happens.
