Speaker impedance changes with frequency, that is why you need a load like a huge resistor of some kind at the ohms you want to test at. Speakers will not give the right reading, maybe if you can measure the impedance but that takes equipment and due to the noise and risk to the sub nobody tests on subs anyway. Another issue is speakers and resistors change when heated, unless you have a giant load that does not get hot you have to measure resistance at temp after the test. You can go to bcae1.com and it is explained under power testing or something.
You can hear a sub clip on a tone it gets a metallic sound, but that does not help you for power testing.
I think I have four 100w ceramic resistors with a fan on them, but I've not tested any large amps yet until I hook up my other power supply. You need to measure that too, what voltage is going into the amp at time of test.
I've always felt like there has to be a reasonable way to test amps like this at home, but I've never heard of anyone doing it in a way that wasn't dangerous or ghetto, or in a way that I felt was capable of producing dependable results. You also need to think about power supply. Even a medium sized amp can draw over 30 amps at 12-13 vdc and thats exceeding the capacity of just about any moderately priced power supply on the market that I'm aware of.
Power supply are expensive and you don't want to be charging batteries in the house, unless they are sealed...which are expensive lol. I have a 10A PS I got free and rebuilt then used a battery with it just to run stuff once in a while in a vented room, mostly just to see if it worked properly. Had a 1A auto charger on it but it eventually died. Then got a 30A PS off a friend and now have another 30A to hook into it. But I mostly do that in the winter so it has sat there all summer. Found with the 10 and 30 my power wire was too small so have some cheap jumper cables I bought to chop up, hope they work with 60-70A. Also got another small PS that might run HU, and have a 10A PC PS but they are 12v not 13.8v, not sure if I can adjust it or not.
Bucket of ATF, messy but effective lol. I've only run around 400rms into it and not for long, it didn't seem to warm up really. Actually mounted them in some old amp case with a PC fan on them lol. I was told they will take a lot more power if you air cool them, but you have to check the resistance right after testing because it will change.
Think I got the resistors from Mouser, maybe Digikey. They are 4" long or so. There are some giant server power supply on cheaper at times. One guy I talked to had three server PS and he used the 5v out for 15v, then turned them down a little to get 14 or whatever he wanted. But he is a real tech lol, has a lot of equipment. I just play around with it and actually want to test some old school amps for fun as well as repairs.