I started to answer this post when it was first made, thinking more qualified people than I would answer. Your amplifier doesn't know how many speakers you have. All it sees is a load (impedance, resistance). So long as you do not exceed the minimum load, your amplifier will be fine. I consider going below a rated 4 ohm load to be amplifier abuse. A rated load, as has been pointed out, is usually more than the actual load; in other words, a 4 ohm rating is probably closer to 3 ohms. An amplifier rated to handle a 2 ohm load, as some are, I consider to be an exercise in planned obsolescence. 4 to 8 ohms is ideal, in my opinion. Higher than 8 ohms results in a diminishing return. Jerry and anyone else drove higher numbers of speakers using a combination of series-parallel wiring and/or slave amplifiers. Even then, especially with the wall of sound, blown speakers were a problem, primarily because of surges in voltage overcoming the resistance. Think of a water hose trying to wash off stuborn dirt, with no luck. If you place your thumb across the opening, you get a surge of power that may overcome the resistance of the dirt, and it washes away. With a speaker bank, the resistance is the speaker impedance, and a surge can blow it if it is strong enough. That's why smart giggers gig with surge protection. Who knows what's going on in some of these dives electrically, especially the ungrounded ones.
Hope that helps a little. Also, with your cabinet, there is no real reason to open the back or leave it sealed for a guitar. The frequencies of the instrument are very directional, and short enough wavelength to not matter. I am surprised that it had insulation in the cabinet. That seems to be more like a bass cabinet, which needs to absorb waves especially in a sealed cabinet, as it makes the cabinet seem larger to the speaker trying to produce long bass frequency waves against the air resistance of the out-of-phase cycle of the speaker excursion.
Gone are the days we stopped to decide where we should go, we just ride...