Not sure why I've read comments that running the satellites through an electronic crossover has a detrimental effect, while it is OK on the woofers/subs? I guess it would be not having to use an additional cable and the cross-over?I would imagine so, particularly with respect to the electronic crossover. Any sonic degradation or artifacts it might introduce would figure to be more significant if they affected the mid-range (and adjacent) frequencies, as opposed to just the low bass.
Would the capacitor have a 6db slope?Yes. If placed between preamp and SET amp, the high pass filter would be comprised of the capacitor and the input impedance of the SET amp, which presumably is essentially resistive at audio frequencies. An RC filter has a slope of 6db/octave once you get beyond the point at which it is 3db down (that point being referred to as the 3db bandwidth). So if you set the 3db bandwidth (which in this context is the same as the "crossover frequency") to 150Hz, then at 75Hz it would be down 9db relative to the upper bass, the mid-range, and the treble, and at 37.5Hz it would be down 15db.
You would calculate the required capacitance by using the formula for capacitive reactance (impedance), Xc = 1/(2 x pi x F x C). The 3db bandwidth point would occur when Xc equals the input impedance of the amp. So you would substitute the value of that impedance (which might be perhaps 50,000 or 100,000 ohms) for Xc, set F equal to the desired crossover frequency, and re-arrange the equation to calculate C. If you use units of ohms and Hertz, the calculated C will be in Farads, so multiply the answer by a million to get microFarads (uF).
The specs you provided for the amps and speakers seem generally reasonable, but I still have a concern about the disparity in power between the two amps. A 400W amp driving 87db panels will be able to play about 8db louder than a 24W amp driving 91db speakers, which is a substantial difference. Even with an electronic crossover and a sharp rolloff, I think you might find yourself unable to use perhaps 1/2 or more of those 400 watts without clipping the SET amp. (I'm assuming the 87 db figure is for 1W input and at 1m, not 2.83V input and at 1m. Given the 4 ohm impedance, if the 87db is based on 2.83V, that corresponds to 2 watts, or 84db/1W/1m, which would reduce the concern).
Together with the fact that the satellites are reasonably sensitive, and the room is not large, that would seem to suggest (assuming 87db/1W/1m) that perhaps a 200 or 250W SS amp (into 4 ohms) would be a more appropriate choice.
I have no particular knowledge of the quality of the Behringer or Marchand (or other) electronic crossovers. But having the flexibility to adjust the crossover slopes over a wide range, as you mentioned, would certainly seem to be desirable.
Best regards,
-- Al