I had owned an Infinity RSIIa which had a similar crossover box.
The parts inside are very poor (for audiophile) quality.
ALL the parts are common bits. Replacing the caps and diodes and resistors is easy for anyone who ever put any kit together.
For mine, i got the box schematics from Infinity, but I KNOW they will no longer do that for any customer.
When I asked, my speakers were only 5 years old, and they had to send a copy of a copy of a copy of a copy, that some tech had laying around! (I had both the crossover scematics and the crossover box scematics, now lost)
Anyway, ANYONE who is handy with a soldering iron can upgrade the parts in that box.
To upgrade the actual circuitry... Hmmm
Anyway, all the box does is extend the mid and low frequencies.
It is just a glorified tone control.