Hi George
You are making an assumption in your last statement: That an impedance value is all that matters.
Unfortunately, the universe is not quite that simple. The type of output stage that provides the "100 Ohms" impedance (not resistance...) plays a huge role in the frequency domain performance, which is ultimately what we care about with music as the signal.
At one single frequency, you may indeed measure 100 Ohms, like say at 1 KHz (a popular choice). But this value is made up of resistance, transistor gain effects, and parasitic capacitance, of which the latter two are extremely sensitive to frequency, and temperature, and are intimately tied to the circuit design and its parts.
One of the goals of an amplifier designer is to minimize these effects through a combination of clever design techniques that have proven their mettle. However, not all output stages have the same frequency response (i.e., impedance at all frequencies). In fact, there are hundreds, if not thousands, of topologies and permutations, and they all have varying limitations. Some are good, some are ok, some are mediocre, and none of them are perfect. Which you get depends partly on design budget, but mostly on the engineer's design experience.
But wait, there's more: it is the same story for the input impedance of the amp or preamp! Then, when you combine them together, they will interact in yet a third (new) way! Especially with complex signals like music.
So, at the end of the day, it's a crap shoot. There really is nothing definitive in this crazy world we live in. With a preamp might better, might not, have to insert a human in the feedback loop and check. But then that simply becomes their opinion, and how definitive is that?
