I have a JTM45 with KT66s, and a cabinet with 2 old ('70s) blues at 16ohms. I've dimed the amp once through that cabinet. It was starting to fart a bit on the low notes, so I turned it down and have never been back there - I have two other cabinets I can use, and now have a Weber mini-mass, so it's not really necessary.
One point - if the OT is tapped at 16 ohms, the rated output should be lower than 8 ohms, which is lower than 4 ohms etc. Anyone know the precise figures for each impedance setting?
There's a difference between "will damage" and "can damage". Clearly it's possible to hurt 15 watt speakers with a JTM45. How loud are you gonna play? You'll be fine as long as you don't push too hard. But of course, you never know where "too hard" is until you actually damage the speakers.