Fluxtone Speakers

[Diablo1]

I don't perceive that the tone of my FluxTone changes as the field coil voltage is lowered; it's not fizzy at low settings.

With the Fluxtone, when you turn down the field coil voltage, does it start to add distortion especially to the low notes?

I don't perceive that such an effect is occurring; no noticeable change in compression etc over the field coil control range.
The Q and magnitude of the speaker's bass resonance does seem to change according to the field coil voltage, such that with the field coil voltage at zero, the bass resonance magnitude is smaller than at full field coil voltage.
I was surprised when I discovered that effect, as I couldn't hear any difference!

I have three vintage field coil speakers, two Jensens and a Rola. All of them behave the same way as the field coil voltage is reduced. In order to cut the speaker volume way down, I get a large increase in compression and low note distortion

Does the B+ for the field coil have it's own PSU?
I'm thinking that there may be some interaction between your amp's B+ and that for the field coil, with the amp B+ sagging at low field coil settings.

Thanks pdf64 for the detailed response. I am using a stand alone DC power supply for the field coil, and it's adjustable from 7-100 volts. The field coils are around 680-700 ohms, and I'm limiting the field coils to around 15 watts max power, like in the original Hammond organ. The speakers sound very good when I turn the field coil voltage up. It's just when I turn the voltage down that they start adding their own distortion and compression. I tested the speakers out before I reconed them, and found the same effect. I am thinking that the magnetic flux loses control of the voice coil when the BL gets small. So Fluxtone seems to have built a better mousetrap than me. I'm curious to figure out what is different, but I certainly don't have a clue.