The reason you don't want higher impedance for tube amps in general is that the output transformer starts feeding back voltage to the output tubes. It's called flyback, but the mismatch has to be pretty significant. Since the factors involved literally range from the amount of wall voltage and the ambient temperature of the room, the math involved gets very serious when it come to calculating precisely which combinations of speakers are more dangerous than others, and as your amp shows, it's not necessarily a hard rule every time, just like you can get away with 1 and 2 ohm loads on solid state amps that aren't rated that low.
However, as Jake's graph shows, speakers have absolutely HUGE low frequency impedance peaks at the resonant frequency and depending on how the cabinet is wired and what the speakers are, these can be astounding, literally over the hundreds in some cabinets. Good tubes are made with this in mind and the order of voltages they can handle from flyback feedback are numbers that start sounding very scary. The rule is to avoid higher general impedances than the amps are rated, but as your PA135 shows, there are caveats to that.
Other things factor in as well. Ported cabinets will have higher, more dangerous resonance than sealed and the type of signal being run through the amp makes a huge difference too: guitar is many orders of magnitude safer than bass or anything with lots of lows, again because of the resonant peaks at the bottom of the speaker's frequency response. You're fine, but I still tell all the kids who have heard the solid state output impedance rules being applied for tube amps that they should avoid higher ratings because there will always be some idiot who hooks up an SVT to an 8 or 16 ohm ported cabinet and cranks the lows only to watch the one or two cheapo old Chinese 6550's (new ones are pretty good stuff) that his guitar player buddy stuck in the amp arc internally and the SVT starts throwing fireballs as it blows up spectacularly: they used to be famous for doing it. Nowadays, the internet and not the grapevine keep players better informed, helping to keep more old tube beasties alive.