4 ohm vs 8 ohm taps

My speakers are rated at 8 ohms and drop to 3.5 ohms through a region in the bass. They always sound best from the 8 ohm taps on either of my VAC amps. I think you should stay with the 8 ohm taps.

Normally the 8 ohm tap would be correct but:
The 8 ohm tap will output 2x the voltage but 1/2 the current as the 4 ohm tap. However speakers impedence is not purely resistive and depending on the impedence vs freq. profile of your speakers, the 4 ohm tap might be better. Try both (as long as you don't listen too loud) to see which sounds best to you.

Yes, it is a try and see kind of thing, but the 4 ohm tap will always have lower impedance which will translate into better damping of the speaker, no matter what impedance the speaker is. This will not always sound "best" and using a 4 ohm tap with an 8 ohm speaker will sometimes diminish the power output. I say "sometimes" because the interaction is complex and many 8 ohm speakers drop down pretty far at a given frequency, as mentioned above.

I use the 4 ohm taps on my VAC amp, and I'm using speakers with a 6 ohm nominal load, that dips to 3 ohms in the lower bass. I've also tried the 8 ohm taps.

If you have an 8 ohm speaker, use the 8 ohm tap. If you use the 4 ohm tap, the transformer will not be loaded correctly and will 'ring' that is to say it will add distortion, which usually makes the amp sound brighter (though not always).

Conversly, if you put a 4 ohm speaker on the 8 ohm tap you just cut the load that the tubes see. For example if the tubes are seeing 3000 ohms 'plate to plate' the impedance just dropped to 1500 ohms. This will cut power (maybe by as much as 1/2) and add distortion! Transformers are called that because they transform impedance and that goes both ways.