Using Adcom GFA-555s as monoblocks to power magnepan 3.3s in active biamp

To Tackle your simpler question:

Take a look at the Stereophile measurements on any speaker.  Regardless of what the manufacturer specifies as its nominal impedance, the solid line in the 1st graph will show the impedance (up and down) varies significantly by frequency (bass to treble is left to right).  The discontinuities normally correspond to port tuning, crossover frequencies, cabinet resonances, etc.  While the nominal impedance (along with sensitivity) should give you some idea of their power requirements, it's not unusual for the speaker impedance to vary from half (or less) to a few times that value.  It is quite common for the lowest impedance to be reached in the bass, where power requirements are highest.

You won't find that graph for SS amps, as their output impedance is usually a fraction of an ohm and much less variable across the audio band.  However, any distortion or clipping graphs will normally climb slowly on the right side of the graph, then suddenly and dramatically turn north.  You don't want to push your amp into this region!

Depending on the nature of the mismatch, the result could be an amp or speaker fuse blown, or a speaker driver or an amp output section.  Your house & neighborhood are probably safe, though!

@petaluman 

"Depending on the nature of the mismatch, the result could be an amp or speaker fuse blown, or a speaker driver or an amp output section.  Your house & neighborhood are probably safe, though!"

That's good to know!  Usually when I do some tinkering and something unwanted happens, it's never small.  I go big!

@allenf1963 If I may comment on @petaluman ’s answer, in terms of Ohm’s Law, let us say you have an old-time filament-type flash light. If you Ohm out the bulb, it shows continuity, just as a speaker coil does, but when you turn your old-time flash light (or ‘torch’ as the Brit’s are wont to call it) on, the filament lights up and gets warm, the warmth is energy (power) being dissipated. The bulb (or more correctly lamp’) is rated in Watts because of the energy (power) it is releasing, mostly in the form of heat, and only secondarily as light. [Light Emitting Diodes (LEDs) emit more light than heat, but that’s another story.]

Now, suppose that this is a magical flash light and capable of illuminating the space in any color of the spectrum and beyond, shining different colors like a loud speaker reproduces different frequencies of the audio spectrum. What @petaluman is saying is that this ability to ‘play different frequencies’ is not without cost. As the wavelengths get longer (that is, frequencies get lower), it takes more power to generate them. For light waves, this means that red light is harder to generate than blue or violet; or, more to the point, that bass is more difficult to generate than treble. In fact, when really low bass notes are generated, unless the amp and speaker are designed for it, the speaker appears as a near short to the amp. Remember, to an Ohmmeter, it IS a short, it is only because the wire is wrapped in the form of a coil, and has AC current running through it at a particular frequency that it exhibits a resistance, or, more properly ‘reactance’ (XsubL). So the speaker coil gets warm and expands, possibly shorting, the fuse, if installed blows, the output transformer (if part of the amp’s design), heats, shorts, and blows, any integrated DSP chip starts complaining and sending messages to the screen before it blows —and the music dies. In our flash light analogy, running the red light all the time burns the batteries up more quickly than light of higher frequencies.

A disclaimer, the flash analogy is used for illustrative purposes only, I have no idea how much energy it takes to produce light of a specific color, although, since red LEDs were produced well before the relatively recent white LEDs I may have it backwards. I was trying to emphasize the longer wavelengths of bass compared to treble.

Hope this helps.

@allenf1963 I forgot the math. Since P = I * V, and since V = I * R, then P = l * (I * R). When you turn the flash light on, and the bulb heats up, it’s resistance increases, and you find the source voltage across the lamp (don’t try this with an LED). If it takes more power to produce red light than blue or violet, then either the resistance has increased and more voltage is required, or the resistance has decreased allowing more current to flow for the same amount of voltage ‘driving’ the circuit. In the case of a speaker’s impedance (XsubL) drop producing the lower bass frequencies, because the speaker offers less resistance at the lower frequencies (XsubL = 2(pi)fL), more current passes through the circuit for the same amount of voltage driving the circuit. (With semiconductors it is easy to enter a ‘death spiral’ in which more current generates more heat which draws more current, which generates more heat, etc. the semiconductor offers less and less resistance until it finally burns itself up). Unless there is a microprocessor monitoring the circuit’s power usage, or appropriate and well-placed fuses in the circuit, part destruction is a real possibility where impedance ‘droops’ occur in the bass region, or if you hook up 4 Ohm (or 2 Ohm) speakers to an 8 Ohm tap (at least in theory, many folk recommend trying your speakers on different taps, just to see if they sound better). In practical terms, it isn’t going to make much difference unless or until you turn the volume up and start to drive the amplifier into clipping or at least start to use significant power (unless there is a short in your speaker or connections).

@oldrooney 

Again, solid info and I'm starting to gain a grasp.  Another simpleton question, in its most basic use, is the purpose of the speaker's crossover then to alter the voltage or signal that is going to the bass, mid, and treble drivers -- to produce the sound signal you want without burning out those individual drivers?  Again, I know this is Kindergarten stuff to most of this Forum, but even though I've been a music fan and around bands my whole life, this was the area that just puts me in a "deer in the headlights" mode.  I know I'm not the only "lurker" in this Forum this puzzles over these questions...I'm just not afraid to admit my ignorance!  😂

Thanks again.

Allen