Defining nominal impedance

Hi jimspov,
    You'll need to try both.  Any speaker has a huge impedance peak at the resonance of each driver and most have some significant dips afterward. So according to Wikipedia, that should mean its lowest dip, my experience is that Nominal should mean, "where the speaker spends most of its operating range in"....
Either way,  depending on where the dips or peaks are at, Your tube amplifier will most likely sound better on one tap or the other. I suspect that you'll hear it right away.  I hope this helps,  Tim

Nominal impedance of the speaker is defined as its design intended impedance i.e. the designer intended it to be 8 Ohms or 6 Ohms or 4 Ohms.

i do see where you got the info that nominal impedance of a speaker is the lowest impedance after resonance....
but take a look at the following review of the B&W 804D speaker whose specs state that it is an 8 ohms nominal impedance. Look at the impedance-phase plots. the impedance curves are nowhere near 8 ohms after any of the 3 resonances!!
So, this flies in the face of the Wiki info you found!! ;(

http://www.stereophile.com/content/bowers-amp-wilkins-804-diamond-loudspeaker-measurements#4g61rRpjW4oTlfGm.97

what is resonance?
dial back a bit - a speaker driver can be modeled as a mass + spring. The cone being the mass & the spider network making the cone springy as you push (gently) on it & acts like a spring. (the voice coil suspended in the magnet also acts like a spring but just to keep it simple I mentioned just the spider network). For this driver there is an electrical frequency, which if feed into this driver, the cone will move back & forth at its maximum allowable excursion. If you measure this you will find a peak in this driver’s frequency response. This peak is called the resonance frequency. Every driver has a resonance frequency.

should you use 8 ohm post or 4 ohm post for your 6 ohm speaker? it really depends on what kind of speaker you have - one like B&W 804D that has some wild impedance/phase plots that require it to be driven by a high current amp OR if the impedance/phase plot is mostly flat thereby giving you the option of driving it with a lower output current amp.
In general, 6 ohms connected to 8 ohm tap - you will get higher output voltage & less current drive. 6 ohms connected to 4 ohm tap - you will get lower output voltage but higher current drive. The amp is more stressed now as (it has to deliver more current into a lower impedance). For most s.s. amps, the output power will increase (into 4 Ohms) but for tube amps the power could drop a little (10-20%) as tries to deliver more power (since tube amps run on the Power Paradigm & P = V*I, if you deliver more current, the output voltage drops & since the amp is stressed delivering more current, the output voltage could drop a little more than proportional/linear; hence the drop in output power by 10-20%).
What does your 6 ohm speaker need? More voltage or more current? If you have any knowledge of its impedance/phase plots you can better determine if you need the 8 ohm tap or the 4 ohm tap.
If you don’t know - try both & see which one sounds better.....

I have a pair of speakers that are listed as 8 ohm "nominal impedance", which might be the impedance at 1000hz, but show a "minimum impedance" of 3.1 ohms, so I contacted the manufacturer for my amp, who recommended I use the 4 ohm tap, not the 8 ohm one.