When an amp puts out 90 watts....

As Zd says, and I just confirmed, it's a tube amp. This is how tubes amps typically spec. Nothing to get worried about. And by the way, only a very few solid state designs with massive power supplies are able to double power into halved impedances. This is NOT the norm.

Rrog: It is the "norm" for me and, because it matters in my system, it's why I have a Krell 400cx driving my Thiel CS6s. My point was that the vast majority of amps do no comply with the doubling of power by the halving of impedance from the incrementals of 8 to 4 to 2 Ohms. Only a handful of manufacturers install power supplies robust enough to do this.