Greetings
Since you are an amplified musician who "uses ohms on amplifiers" I'm confident I can help you understand better what is going on but perhaps unfortunately for you it is not as simple as Ohm's Law. When you say you "use ohms" I'm betting you are referring to speakers and labelled connections on the back of your amplifier.
Let's start with basics. Ohm's Law is a beautiful expression of the linked interaction between all the variables in the most basic of circuits - A driving force or pressure (voltage), a sort of volume in motion making work possible (electronic current), and a resistive load, doing the work. Change anyone of these elements and the others change too. Ohm's Law let's us calculate what those changes will be.
Now for your amp/speaker system and how it varies from a basic circuit. A straight wire presents a fixed resistance at a given temperature based on it's composition and also it's geometry - ie: a solid wire will have a different resistance than a stranded wire, largely because electrons tend to not travel through a wire but over it's exterior surface, it's "skin", if you will. More surface area translates into more electrons available thus reduced resistance to flow. This last bit is oversimplification but it should do for the basics.
How your speaker system is different is that the load, is a wire (actually in most cases a very special kind of edge-wound ribbon wire) that is wrapped in a coil. This coil which is an electromagnet when current flows through it, surrounds a permanent magnet. When the poles align it moves one way. When they are in opposition it moves the opposite way. Since the coiled, electromagnet is connected to some sort of diaphram, usually a cone, this structure acts as a piston creating oscillations in the medium, usually air, that mimic the oscillations of the current flow in your amplifier, which mimics the oscillations in your instrument. Thus the speaker can be said to reproduce the sound, the frequency and amplitude variations of your instrument.
The problem is that the coil not only has the fixed resistance of the same wire laid straight, but once wrapped into a coil and surrounding a magnet the system becomes considerably more complex. Without getting too deep just yet, suffice it to say that this form of resistance (called inductive reactance or simply, impedance) varies with frequency.
So you may have a speaker system that is labeled 8 Ohms. If you measure it with a DC voltage as all basic VOMs do, you will perhaps be surprised to read it commonly somewhere between 5 and 7 Ohms. The 8 Ohm label is a functional average. An 8 Ohm speaker may be at 20 Ohms below 50 Hz, which is one reason why it takes more power and larger speakers to reproduce bass frequencies at a given sound pressure level, commonly and incorrectly referred to as "volume".
I could write for days on this subject and some people actually have, but I should probably stop here and await some feedback from you. I hope this makes sense to you because it is truly a peaceful pleasure to understand how your instruments actually work and that often includes amps and speakers since in musical instruments exact reproduction isn't always desirable. Sometimes that esoteric "color" and "feel" is more important. These too can be expressed in mathematical terms by people who comprehend both the Art and the Science of musical instruments. I encourage you to continue in your quest to understand yours more fully. An artist should have intimate knowledge of his tools.
