Nothing to do with watts. I’ve seen and experienced low powered amps taking out speakers, usually the voice coil, because of a distorted signal. I suspect this may have been the case with the OPs 135w tube head. When I’ve seen it happen, it’s usually been caused by a tube head or distortion effect - nothing to do with power handling. I’ve know this happen with an Orange 120 and an Aguilar 412 - two speaker coils burned out.