If I understand correctly, the Theater 4 Pi works great with SET's and such; the Theater 3 Pi does not work well with tube amps (especially not SET's); and the Stage 4 Pi works pretty well with tube amps down to about 10 watts. Not having read every post you've made on the subject, from what I have read I gather that the issue at hand is the woofer's impedance at resonanace.

The Delta 15 in the Theater 4 Pi has a Zmax of 52 ohms; Delta 12LF in the Theater 3 Pi has a Zmax of 93 ohms; and the Omega Pro 15 in the Stage 4 Pi has a Zmax of 122 ohms.

I would have thought that a fairly low Zmax would be desirable for a tube amp, but evidently the high Zmax Omega 15 works better than the medium Zmax Delta 12 LF. I'm probably taking bits and pieces out of context - could you show me the big picture, or point me to a source?

Thanks!

Duke