A 1 watt frequency response test will tell you a lot as well. Most highly stable vacuum tube amplifiers can be very flat up to 20 kHz, but will start rolling above that, usually being down by about 1 db at anywhere from 40 kHz to 60 kHz. Response that has not started rolling off by about 75 kHz is usually a recipe for stability problems in high fidelity equipment.
Dave
Dave