[Charles]

There is no question that miss matching the impedance will alter the bass. That's why it is important to use the nominal impedance supplied by the manufacturer as a guide. For example if it is 5-8 ohms use the 8 ohm tap. If it is 2-4 ohms use the 4 ohm tap. This is for modern day Mac amps. Older Mac amps were much more current strapped. Modern day Mac amps have plenty of current if you follow my suggestions. Then it is a matter of efficiency. The more inefficient the speaker the more watts you will need to get the best most relaxed sound. I can't speak to tube amps as I haven't owned a Mac tube power amp in many years.

Also, in my opinion Mac has taken a very intelligent approach to the relationships among current, impedance, and watts. The brute force approach would be a D'Agostino approach. His amps have tremendous current capacity at any impedance but produce less watts the higher the impedance. For example one of his rated at 300 watts at 8 ohms will generally produce 1,200 watts a 2 ohms. But at 8 ohms it will produce 300 watts and a little more (dynamic headroom) before it clips.

What this means is an amp with a mega power supply capable of producing many hundreds even thousands of watts at low impedances. A modern day Mac amp like a 601 or 1.2KW has plenty of current if you don't miss match the impedance. It is impossible to miss match the impedance with an D'Agostino amp.

Mac amps should run very cool even when stressed. If your amp is hot you may have an impedance miss match which I think inadvisable.