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Romy,
I might have overlooked something on your site but I understand that your ULF amp is driven directly by the preamp line stage. If that is the case, you may want to consider an old technique for integrating subwoofers and drive your ULF amp with the attenuated output of the midbass amp. Perhaps this is an idea you considered and discarded but sometimes it helps to state the obvious.

Frequently with a deep bass speaker we want to bypass the low frequency limitations of the main amp and drive a separate bass amp directly from the pre-amp. But in your case, the midbass SET amp is optimized for the low bass so using it to drive the ULF solid-state amp might help restore some tonal system integration.

This technique should also help to improve the phase alignment of the ULF and midbass speakers since the ULF would see the phase variation of the midbass low-pass crossover and any phase variation from the SET amp/transformer. You may need to adjust the ULF/midbass crossover afterwards and pay attention to any phase inversion of the amps.

I hope people reading about your progress appreciate that a highly refined integration of your ULF and midbass sections is not a straightforward exercise. The midbass uses a transformer coupled tube amp, the ULF a direct coupled solid-state amp. The midbass is horn loaded, the ULF a direct radiator. They are positioned on opposite sides of the listener and their traveling waves moving in opposite directions. The pass bands are narrow allowing the primary high-pass phase/amplitude responses of the loudspeakers to mix with the responses of the crossover filters in unintended ways. Even if the ULF output is not supposed to overlap the midbass output I suspect it does so in practice - and of course the ULF distortion spectrum and resonances surely factor in the midbass if they are audible at all. And as I mentioned in an earlier post, there may be unwanted crosstalk effects between left and right channels because of close placement of the midbass horns. Be happy the results have been so pleasing and the only need is fine-tuning. Even from afar I have found this a provocative technical challenge.

---Robert

P.S.:  If you are going to reposition your ULF towers you might consider facing them away from the listener seat in an effort to reduce by redirection any higher frequency distortion or resonances that may have some (small) beaming to the front. The frequencies you want the towers to reproduce are very omnidirectional so there may be just a slight benefit to turning the towers around fully or even somewhat from the listener.

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