I'm hardly a guru. The 90dB number seems more than a bit on the optimistic side. I could not find that in any documentation that I have. No efficiency rating that I could find. My personal opinion is that they are the least efficient speakers I've ever owned. In my experience ,901s need gobs of power. They demand it. m1.5t is a good match. Before they changed the spec fairly late in life they were power rated at "unlimited in non-commercial applications". The brochure indicates the amp can provide as little as 10w.... i think that's a joke?