Where is the spec for BTU's ?
Haha. As it happens, Krell do list BTUs (no apostrophes for plural, come on) for Evolution series, which I consider peak Krell (they didn't do BTU for all their products, sadly). We have the first series, and the e-series, which have an additional low current standby mode. They are class A, but with proprietary plateau bias so they don't run hot until they need to. The peak output is instructive (and pretty much what standard class A would put out constantly) and also an indication why we don't have many 900 watt class A amps without power management on offer.
Here's original series 600/1200/2400 and 900/1800/3600 watt (into 8/4/2 Ω) monoblocs (left and right respectively). Remember to multiply by two for stereo.
Here's the e series (they are the ones to get, obviously).
I've spent a bit of time listening to the baby 400 monoblocs driving Focal Grande Utopia. They do ok.