As I understand it, the by-pass cap allows the primary cap to discharge faster which results in cleaner more detailed sound.
The Sonicap Platinums I used are no longer available in the size needed for a by-pass cap. Currently the by-pass cap of choice around here is the Miflex KPCU (copper/paper in oil). Danny has them in stock, give him a call.
I’ve built X-LS Encores with many crossover levels, from the original pre-built PCB crossover to the version in these speakers. Every upgrade makes a difference.