4-ohm setting with 8 ohm speakers

Okay, I see a lot of erroneous info flying around in this thread. A 4-ohm load is a lower impedance and thus a tougher load for an amp to drive (not easier).

Some confusion creeping in here involves the typical solid-state vs tubes. Typically, solid-state amps have more power driving into lower impedance loads, in fact, many of them double power into 4-ohms (this does not mean they sound better driving 4-ohms). Tubes, on the other hand (which is what we're talking about here) are typically optimised to drive 8-ohm loads, with some optimised for closer to 4 ohms.

This means that many tube amps will deliver a bit more power into 8-ohms than they will into 4-ohms. In most cases, the actual power difference is small, unlike solid-state.

That said, if you use a tube amp's 4-ohm taps to drive 8-ohm speakers it does increase damping factor and decrease distortion. In some cases, this can sound "better" to the listener, and in other cases it can sound "dryer" and more like solid-state. Sometimes the little extra distortion and looser bass is preferable to some listeners, because it usually gives a sweeter, though less focused, sound.

In this particular case, the results probably have to do with the tube amp being optimised to drive closer to a 4-ohm impedance coupled with this particular listener's personal bias. And of course interaction with the impedance curve of the particular speaker will come into play. This is why amplifiers can sound different when driving different speakers.

And yet, nobody has said a single word about reactance. This is implied when taking about impedance. Tube amps do not like capactive loads. period. I hope I got that right.
A 'flat' impedance curve? I've never seen one. If their are any crossover components at all, the speaker should 'swing' from capactive to inductive at some point, shouldn't it?
I think damping may be called on for Plato's above comment about 'looser bass' when using 4 ohm taps. With the whole amp OFF, how much resistance would I measure on 4 ohm or 8 ohm taps?

Other than that? Plato makes sense. Use whichever tap you like. I'd add that if the amp runs too hot while using the 4 ohm taps, I'd reconsider either amp or speakers.

Magfan, I think you read my comment wrong regarding me saying you'd get "looser bass with the 4-ohm taps." Perhaps I didn't make it clear but the 4-ohm taps (compared to the 8-ohm) would increase damping factor for tighter bass. Distortion would also be less using an 8-ohm speaker on the 4-ohm amp taps. I said the bass would be looser using the 8 ohm taps, and that the 8-ohm sound may be sweeter though less focused/precise, but that some folks might still prefer it that way.

What Plato describes regarding the bass is the effect I get when light loading my Music Reference RM-10 MkII. The loss of power is minimal as well, roughly 20%, so it runs at around 28 watts versus 35 watts. That's pretty negligible with my speakers since they are fairly efficient with a very smooth impedance curve.

Now my Music Reference EM-7 v12 amps can be configured for 3 ohm or 12 ohm output and I have tried both. Again, the lower output exhibited tighter bass and less distortion. However, with these amps I prefer the higher output.

And to complicate matters a little bit with what will likely be a footnote to the discussion, it should be mentioned that output transformerless (OTL) amps actually increase their output power into higher impedance speakers. Atma-sphere amps are the most well-known OTL amps although there are also OTLs from Tenor, Transcendent (incl kits) and others.

This makes Speltz autoformers particularly helpful with these OTL amps, although as the Speltz site testimony makes clear the autoformers can improve sounds with other amps as well.