All this ado about not much......

First of all just float the scope. For the signal levels and impedances you are dealing with just don't worry about it. Now of course this requires diligence to keep Murphy's law from being enforced, but in my 30+ years on the bench I have determined that it is usually better not to have things grounded on the bench.

What's more you don't even have to do that. This is all fine for an academic discussion but in reality there are a few things to consider before getting one's bonnet in a beehive. At least 99.999999 % of the time the same thing is going to be on the + and - side of that speaker, just inverted. If not you likely have a problem in the output stage after the phase splitter. If it is OK from that poit you can measure either speaker wire and derive anything you want, just do a little mental math.

Now since we are such the academic group here, there are some things about measuring speaker current which makes this inductive pickup method not so desirable. Unless this is some wierd tube (valve) amp with no damp(er)ing factor at all, the output is a voltage source. That means the voltage at the output is supposed to follow the voltage at the input, relatively regardless of the specific current drain at any given frequency. On the other hand the speaker is a complex load, in a bass bin or subwoofer it will be inductive with some resistive component. This affects the current drain at varying frequencies and makes measurements of amplifier perfomance near useless.

In practice the inductive component of the woofer(s) partially copensates for the fact that the radiating surface is much smaller then the wave it radiates. The lower the frequency, the less efficient it is, however the lower the frequency the higher current the voice coil draws. It somewhat offsets..

Tell you what, test the +/- theory just by getting two resistors, say a thousand ohms or so and put them in series across the speaker wires. Stick a scope at their junction and crank that baby up. If you see relatively a flat line and hear sound that means the output stage is OK and you can proceed. Now just scope one side or the other, doesn't matter. If you need to calculate output power just double whatever voltage you read, or quadruple the calculated wattage.

When dealing with this type of audio, I recommend aa 1 Khz square wave. It can tell you alot about frequency response. Any decent amp should reproduce it quite faithfully, at least at a glance. To get an idea how to read it, put it through an audio equalizer and watch the effect. You will get the idea of what it looks like when low frequency response is lacking, or whatever. Looking at the flat parts of the output waveform is almost like looking at a frequency response curve in reverse. That is with the highest frequencies showing at the beginning and the lowest at the end. Looking for these characteristics with the scope should allow you to isolate which stage is causing the problem.

If this is like most of this modern junk it probably has eighty-one digital preset EQs that are modelled after some blues player's amps from 1956 or some ****, but if the response deficiency is in all of them, the problem is most not within that little universe/mess. It would also be nice to know if the problem came on all at once or gradually.

People who write regulations are not techs. I do not want anything earthed on my bench. Nothing. I have good reasons for that which are numerous. I can understand the concerns of people who believe that every square inch of this world should be safe enough for a four year old to play unsupervised but I do not espouse that bull****ing****. This is my bench and keep your hands off. If you happen to get shocked, see rule number one. In fact you guys are lucky with the single ended 240 volt mains, over here we have to deal with voltage doublers which make floating anything much more exciting. I try not to do it, but I will do it when needed. Suffice it to say that isolation transformers are needed more in the US for a couple of reasons.

I would like to float everything, even the antenna/cable lines in the shop. Put an isolator on each one. In my plug mold I not only didn't connect the grounds, I cut the green wires between each and every outlet. Of course this was at my shop, where I am now doesn't have that luxury and I am too lazy to do it.

Maybe I am crazy, but it hasn't killed me yet.