Hey, a couple points to your post:
I bought my RM50 for $540 (shipping included) with 3 modules on ebay. Probably could have been cheaper, but I turned one module for $120 and so it cost my $320 - heck of a deal!
For measuring the speaker impedance, it is always done with the speaker disconnected from the amp. From electrical theory, for maximum power transfer, you match the load impedance (speaker in this case) to the generator impedance (the amp's output transformer in this case). Actually you match it to the the complex conjugate of the impedance but it can be simplified here.
So, for an 8 ohm speaker, connect it to an 8 ohm impedance secondary on the transformer. The same for other impedances (4 to 4, 16 to 16). A 16 ohm speaker connected to a 16 ohm output impedance looks like 2 16 ohm resistors in parallel = 8ohms.
Additionally, the transformer is designed to be damped properly with the right load. If you put the wrong load (ie: wrong impedance) there will be ringing in the transfomer - translated as a bad frequency response characteristics and/or bad transient response.
Not sure about this point, but my guess is that the ringing could induce oscillations, which at high enough voltages could damage components leading to damage to the amp or heat in the transformer causing damage. Let me check on that point
Way more than you wanted know :wink: