Thank you for the lesson, very useful. I was a Ground Radio Repairman in the USAF. My impedance training was focused on matching antennae to transmitters (along with other factors). A visual "test" of radios, like a big SSB, was the size and number of capacitors. We paid attention to impedance matching. Along with the characteristics of the antenna termination... open, capacitance, resistance, inductive , etc. I've forgotten 90% of what I learned (it was so very long ago). But for my humble needs, if the manufacturer lists power output to the speakers as doubling, or nearly so, from 8 ohm to four and again to two ohms, it is a high current amplifier. If you open the hood, large transformers, and large capacitors are a visual clue. Do not get across the terminals of a large cap and ground without discharging the cap. The potential (voltage) can kill, which explains the "No User Serviceable Parts" stickers.
Not just any speaker will perform its best with just any amp.