Many years ago, I taught myself to evaluate a speaker's tone potential by scratching its cone with my fingernails and lightly tapping its surface, so I am very up for this sort of emperical exploration.  I might also make a very light "tool", some sort of "sliver", that I could rest on the surface and watch it move.

I have been very lazy about pictures, so far. But I do have a digital camera, and I want to take advantage of the insight you are offering, so I'll see what I can do.

Best regards,
Paul S

After another couple of weeks in X/O Hell, I think I at least have the notches where I want them.  Yes, I added another notch filter, because the Lowther response is so ragged that using a simpler curve results in bringing down frequencies I need to leave alone.  I also used Martin King's response curve and impedance plots as a basis this time, so attenuation levels are better, as well.  This time I also tried to correct a baffle step attenuation from last time, and that worked perhaps a little too well, with the low end (amazingly...) PERHAPS a trifle too loud.

All in all, the DEBZXs do sound much better with my now rather complex X/Os than they did with the stock X/Os.  Again, these X/Os and filters mitigate response level anomalies but they do not correct mechanical and electrical problems of the drivers, including the Lowther.  However, like I have said many times before, I can no longer listen at all to the "raw" Lowther, so if only in this sense the experiment is a success.

I am currently weighing the WAF of spending even more time on this, to take out the LCR resistors and sub in some pots that will give me a range of resistance, so I can play with this stuff a little more without having to R&R resistors every time I want to fiddle with it.

The DEBZ speakers now make a HUGE, rich soundfield, and the limiting factors are the drivers, themselves.  I am also attempting to address these more fundamental issues, but "progress" as such does not so far warrent further mention of this gambit.

Paul S

Against long odds, I got another chance to do some more careful listening yesterday.  On a hunch, I put the Fluke on the ML2s, and, sure enough, the left channel had drifted up to 340mA and the right had moved up to 300mA.  Once I dialed the operating points back to where I wanted them, I finally got to hear what I had/had not accomplished with the DEBZs and my "Powerful SETs".  I was a little anxious to get in a good listen before the new caps go into shock for two months, and I have to say I have accomplished most of what I set out to do.  Sure, I simply abandoned ULF, but I will not feel bad about this until someone else comes up with a way to get true FR at live volumes from one amp.  Right now, I'm good to the high 30's with one amp, "approaching" live volumes; loud enough to get the message, from classical music.

Since no one but me uses full-blown, original DEBZs, this is not so much an advertisement for this speaker design as it is my own attempts to get FR from one SET amp per stereo side, just like the thread header says.  Even if I keep playing with contours, I have fed my curiousity about FR with one amp, and I have in the process pushed the venerable Lowthers a good deal farther than I thought they would go.  In doing all this, I believe I have come up against and gained a better understanding of the stock electrical and mechanical limits of the drivers that comprise my speaker system and, in the process I have also, finally, pushed the effective limits of the ML2s, as far as these particular speakers, at this particular point, are concerned.

Not to sum, but to recap and re-frame:  I am pretty sure at this point that the range from, say, 70 Hz to high enough can be done quite well with a single powerful SET and 3 -4 "way" direct radiators, with 2 amps per stereo side.  To gain dynamics and clarity over what I have now would require "better drivers" (as opposed to significantly different topology), and I have actually started working on this, very slowly.  In the meantime, my DEBZs work better than ever now, by a fair margin, still a single SET amp per side.

Paul S