I don’t quite understand why someone would use a speaker, or whatever, that is less than the best, sometimes it’s to meet a price point, other times I’m sure it is for other reasons.
For me there are pretty much 2 paths. The Vandersteen path is an attempt to make everything sound good, regardless of how poorly recorded it is, etc. This is done by going dark, muting the highs to be sure, and with some really poorly, IMO, gear, everything. Vandersteens mids, if memory serves, are pretty awesome, and the bass too is good, but the highs sound like a great set of speakers with a towel over the tweeter. IME, that type of gear should be left alone. If the bass is weak, it’ll take more than caps to fix it. I am generalizing here, but it tends to be more often than not the case.
The other side of the coin is the hi def/ hi rez side. Attempting to get every drop of detail possible out of the music. This is where I live, and it is certainly more challenging. You can get very transparent gear, even some bright gear, mate it with Vandersteens and it’ll still sound pretty good to folks seeking the Vandersteen path. With my system, one bright piece of equipment will drive you crazy, you just can’t listen to it. OTOH, pleasing colorations like tubes are also more apparent, and generally more enjoyable. Cables are more critical, teflon caps are a plus because you can only afford to use well designed well balanced equipment.
As for the crossovers in question, my experience with Mirage speakers, limited though it be, is that they are reasonably, to very transparent speakers. Therefore, it isn’t unlikely that a remote crossover rebuild will turn out well, but that is an opinion formed out of generalities, so it could very well be as wrong as wrong can be. The safest bet is to work on it with the speakers available so that you can immediately make corrections if need be.