Word of advice; when testing any audio amp with a fault, NEVER use directly connected speakers. If the amp puts DC on the speakers, they will be, as the eponymous TV show said, Destroyed in Seconds.
Instead, for solid state amps it is generally quite OK to test with no load. Valve (vacuum tube) amps should not be run unloaded, but then they would never put DC on the output due to being transformer coupled.
If doing any volume of amplifier repair work, what you really need is a test load, or pair of test loads, consisting of a suitable power resistor, plus a second resistor of higher value (100R or so) with a monitor speaker in series. That way the monitor speaker samples a fraction of the output, and if full DC appears on the output you may lose the test resistors but that's a lot less costly than losing the testbench speakers.
RS metalclad power resistors on a decent heatsink with a thermal cutout are ideal. Another option is a pair of old fashioned power rheostats, if you can find a pair of 10 ohms or so. (The rheostat is neat as the wiper can be used as a monitor volume control.)
Bear in mind that an amp fault may make the test resistors very hot, so beware of fire or burning yourself. Thermal cutout advised for this reason.
LEDs to indicate the presence of DC or a relay to disconnect the amp automatically might be a useful addition, but the basic two-resistor per channel and cutout arrangement is perfectly good.
IMHO in the tests, the Yamahas are by far the best (No surprise there) The JBLs are excellent if a little bassy, but at least the bass is clean. The Rokits are dull-sounding with a serious bass boom. (You can't tell one bass note from another)
In a studio you don't want over-strong bass anyway because that would predispose you to mixing 'thin' tracks. You want something that tells you exactly how much bass and treble there is, no more no less. The Yamahas do that.