impedance question and tubes

My speakers have a nominal impedance of 6 ohms and range from a high of 11 ohms to a low of 4.5 ohms. My tube amp has both 4 ohm and 8 ohm taps. What does conventional wisdom say about which taps to use. I have found listening to be confusing: with the factory tubes in the amp the system sounded better on the 8 ohm taps: more balanced; the 4 ohm tap was bass heave. BUT, with some nice NOS tubes in the amp which opened up the top end I now seem to get a fuller and more complete sound on the 4 ohm taps. Is it fine to let my ears drive the decision? Also, theoretically, should one set of taps be louder than the others? Thanks for your help.
Hi Dan,

I think that conventional wisdom would be to just go with whatever tap works best for you.

Your amps may not be giving the most possible power at 4 ohms, but if you have the headroom in terms of power, this may not be a problem. I have noticed that Music Reference often recommends running speakers from a lower impedance tap. Roger at Music Reference refers to this as light loading.

Hope this helps. Other more technically competent folks can speak to the potential advantages (or disadvantages) of light loading.
Thanks Roscoe.

This amp has a lot of power (Copeland 506). 90 watts per channel. Speakers are not the easiest load at 87db. But it is plenty loud with out any apparent lack of reserves.
Since speakers often do not have linear impedance curves, you do often have to try other impedance taps to see which offers the best performance.

'Light loading' can be good for the power tubes but can also introduce more harmonic distortion, which will be interpreted by the ear as 'richness'. This is because if the output transformer is not loaded down enough, it can ring (distort although not due to saturation). So there can be tradeoffs as any extra distortion will obscure some low level detail.
Thanks Atmasphere, this is very interesting. I will listen again with some of this in mind. Very helpful!