Bypass capacitors. In addition to the Multicap RTX, I have tried either .1 µF or .01 µF bypasses from Sonicap (Gen I and the Teflon® Platinum cap), Hovland, AudioCap, and Wima. In every case the results were similar. Let me digress for a minute or so (depending on how fast you read) and talk about crossovers.
In the simple first order high-pass crossovers we are using for this capacitor comparison, the value of the single capacitor, together with the impedance of the loudspeaker driver, determines the crossover frequency. An 8 µF capacitor with the 4 ohm impedance of the Magnepan quasi-ribbon tweeter gives a crossover frequency of about 5000 Hz (1/2πRC). A smaller value capacitor (for example, 4 µF instead of 8 µF) will raise the crossover frequency. If we used only the very small value .1 µF capacitor without the 8 µF, we would hear nothing because the crossover frequency is now almost 400,000 Hz. Even a dog would not hear it!
So why use a bypass at all? There are actually components of very high frequencies in some audio waveforms. Some are high order harmonics. If you think of a square wave, the right angles at the top of the wave are extremely high in frequency. Sometimes there are high frequency components in very fast audio sounds, for example, the instantaneous tap of a drum stick on a cymbal. These are the sounds that should be "helped" by including a small-value bypass capacitor in a high pass crossover.
Well, I don't think so. If the 8 µF capacitor blocks frequencies below 5000 Hz and passes frequencies above 5000 Hz, why do we need what is actually another crossover for the same tweeter, but operating at frequencies already passed by the big cap? I am sure engineers have a very good reason, and a couple of them have tried to educate me on this subject. I respect the science and electrical theory on this subject, and my technical background helps me to understand it fairly well. But there is one small problem: the bypasses all sound bad! They add a quality that at first sounds like an increase in air and detail, but after a couple of hours becomes an intrusive harshness and discontinuity in the upper treble. Remove the bypass: all of the detail is present but without that grating and annoying sound. The high frequencies are cleaner, smoother, and much more enjoyable. It doesn't matter if the bypass cap is Teflon®, polystyrene, or common polypropylene, the results are very similar. And to be avoided.