4 of those 2.5-ohm "coffin"-type resistors? They are probably wired in series/parallel, but do you know the net resistance of the bundle? Could be 10 ohms or could be 2.5 ohms. Neither value is gonna make your OTL any happier than mine was before I chucked the crossover. (Really, give all credit to Will; all I did was follow his lead because I was so unhappy with the sound I was getting from my 845PXs.) You are definitely better off with 12-14 ohms than 10 ohms. However, I am a little surprised at the values; they are identical to what I was using with my "old" toroid (10 ohms/36uF), before the big modification (removing the crossover entirely). The -3db point of the high pass filter is inversely related to the product of R times C, where C is in Farads and R in ohms. For a 10-ohm resistance with 36uF, I calculate a crossover point of 440 Hz. I would have thought they could use a much lower crossover point with the new toroid, but I bear in mind that Dr. West is very conservative in his design so as to favor reliability. From my experience making measurements, the intrinsic impedance of the speaker is much higher than 10 ohms at all frequencies up to somewhere between 2kHz and 5kHz. At 5kHz, it's about 8 ohms for my 845PXs. (Bear in mind, measurements were made with NO crossover, driving the SL factory bass transformer and the full-range Australian transformer in parallel with each other, directly with an audio frequency generator.) So, as you can imagine, when you interpose the RC network, at low and mid frequencies, much more of the energy is going through the resistor than through the speaker (R is smaller in value than speaker Z at those frequencies, so R dominates). This may be why some guys on SLOG discovered long ago that very high wattage resistors seemed to sound better. There is almost no limit to the wattage ratings they tried, more than 100W, even. Before my epiphany, I was using an enormous 10-ohm resistor hand-made by one of the British contributors to SLOG. It could tolerate at least 500W. (One benefit of bi-amplification is that only the treble amplifier then "see"s that R in parallel with its output; the bass amplifier drives the bass transformer only through the inductor. Thus paradoxically, your SS amplifier might work best on the treble (because it can handle the power demand and the low impedance created by the R, with your tube amp on the bass).
If you go up to 20 ohms, the crossover point would be ~220 Hz. (The beauty of the equation is that once you know the values for one set of parameters, you can calculate in your head what will happen if you change R or C. If 10 ohms and 36uF give you 440Hz, then 20 ohms and 36uF give you 220Hz, etc.) I would have thought that the new toroid would be quite happy at around 300 Hz, not too far from where you are at if you are using 14 ohms now.
There is no difference between bypassing or removing the brilliance control and setting it to zero. But I preferred to remove it; it's a cheap part that muddies up the signal path. You will hear an improvement. No, the speaker is not too bright without it, in my opinion. The easiest thing to do is to re-solder the input and output wires to the brilliance control together on one binding post of the control.
As far as impedance variations of the speaker per se, the ESL can be thought of as a giant capacitor. It has very high impedance at very low frequencies and then gradually falls off in a fairly linear manner to very low impedance at very high frequencies. I wrote here earlier that in my opinion, very low impedance at 10kHz or 20kHz is not a real problem, because the energy requirements are very low at those frequencies. What makes for problems is the circuitry that comes before the panel, e.g., that 2-ohm impedance dip at about 1kHz that came with the old toroid due to its crossover. Nearly every company does something to make ESLs more friendly to SS amplifiers, which in turn makes it hard on tube amplifiers.
If you go up to 20 ohms, the crossover point would be ~220 Hz. (The beauty of the equation is that once you know the values for one set of parameters, you can calculate in your head what will happen if you change R or C. If 10 ohms and 36uF give you 440Hz, then 20 ohms and 36uF give you 220Hz, etc.) I would have thought that the new toroid would be quite happy at around 300 Hz, not too far from where you are at if you are using 14 ohms now.
There is no difference between bypassing or removing the brilliance control and setting it to zero. But I preferred to remove it; it's a cheap part that muddies up the signal path. You will hear an improvement. No, the speaker is not too bright without it, in my opinion. The easiest thing to do is to re-solder the input and output wires to the brilliance control together on one binding post of the control.
As far as impedance variations of the speaker per se, the ESL can be thought of as a giant capacitor. It has very high impedance at very low frequencies and then gradually falls off in a fairly linear manner to very low impedance at very high frequencies. I wrote here earlier that in my opinion, very low impedance at 10kHz or 20kHz is not a real problem, because the energy requirements are very low at those frequencies. What makes for problems is the circuitry that comes before the panel, e.g., that 2-ohm impedance dip at about 1kHz that came with the old toroid due to its crossover. Nearly every company does something to make ESLs more friendly to SS amplifiers, which in turn makes it hard on tube amplifiers.