4 Ohm vs. 8 Ohm, what does it mean.....

In almost every aspect, performance of an amplifier is "better" at 8 ohms that it will be at 4 ohms. Distortion is typically lower and more linear across the frequency range, bass will be more controlled i.e. "damped", less heat to build up or dissipate, etc...

Only problem is that higher impedances make it harder to transfer large amounts of power and current. While tubes can get around the power transfer problems as they tend to like higher impedances better, they still won't deliver the high current necessary to keep an "iron grip" on a woofer. If one can keep a reasonable impedance ( 6 - 10 ohms ) without a lot of reactive swings as frequency changes and do it with high sensitivity, you would pretty much have the perfect load for an amp. Whether or not the frequency response, transient response and dispersion characteristics of that speaker would work for you may be a whole 'nother ball of wax though : )

Unfortunately, many of the speakers that i personally like and own are both low impedance and low sensitivity. This makes for an up-hill battle all the way. There are only two cures for that i.e. either throw them out and buy different speakers that are a lot less finicky or run amps that are built like tanks. Guess which path i took ??? : ) Sean
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Sneaky follow on--what does it mean that McIntosh has always given its' ratings (even back in the 60's)at 2, 4, and 8 ohms? I assume its' a sign (guarantee?) of linearity. I run a 7270 and 7300.

In practical terms it means you are going to have major problems if you use an amp designed for 8 ohm loads with a 4 ohm speaker.
I had a relatively low powered amp (30 wpc class A, dual mono) which sounded real good. I decided to upgrade my speakers and bought a pair of Focus .7s. I was constantly kicking out the protection curcuitry in the amp. I brought the amp into the shop and had it checked out. They said there was nothing wrong with it but as soon as I began listening it did it again. Next I brought in the amp, pre-amp, and speakers along with their respective cables. The guy checked each piece out and didn't find anything wrong. I don't think he ever set everything up and listened to the system as a whole. After years of this I ran into a friend in the HiFi business in Milwaukee and explained it to him. He immediately said "it sounds like you're driving a four ohm speaker with an eight ohm amp."
I bought an amp capable of driving four ohms and never had another problem with the speakers.

Bill, Mac uses output transformers on their SS amps. As such, the amp sees a relatively consistent load regardless of the speaker it is connected to. That is, so long as the "most correct" tap is selected for that individual speaker. There are problems associated with doing something like this ( other than high cost ) though and that is why most other manufacturers avoid it. Even the best sounding tube amps ( like Atmasphere's and probably Tenor's ) don't use output transformers.

Using an output transformer is kind of like running hundreds of feet of zip cord speaker cable after working so hard to preserve the signal through-out the entire chain. It does lend a specific sonic characteristic to the signal though, and if that is your preference, so be it. I know what i like in a system and i set them up accordingly. I wouldn't expect anyone else to do any differently.

Nrchy, it sounds like you picked a speaker that was of low impedance and had a somewhat reactive phase angle. I'm sure that your 30 wpc amp could have dealt with the speaker IF the protection circuitry was not set quite as sensitive to triggering. This might have involved occasional clipping from the amp though, which happens more commonly than many folks would like to think. Whether or not the amp was a suitable match for the speakers without the protection circuitry would still remain to be seen if that approach had been taken. The fact that the manufacturer selected the trigger point on the amp that he did and the tech verified proper operation tells me that it was probably not up to driving the speakers in the manner that you wanted to use them. Sean
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I know that this is a gross generalization but I usually find that speakers with lower impedances have better bass.