Hi
I am in the middle of the same thing right now.
I have followed the advice of some of the experts on this forum and purchased a Crown K2 amplifier. It has plenty of power, but importantly for home use, does not use a fan, so should be dead quiet. I have actually yet to hear it as it was an eBay purchase and I should collect it at end of month. It cost £420, which seems to be the going rate for working models.
Re the actual active crossover, I am getting one of the members to build me one. I am hoping to use it both for the 4343s and for NS1000Ms, so that the electronics stay the same, regardless of the speaker system in use.
There is plenty (too much actually) on the forum about crossovers. All agree that the bi-amp switch should be dropped. I plan to rebuild what is needed for bi-amping and remove the switch completely. What I have not found is background information on converting the 4-way passive crossover to a 3-way, for both the 4343 and the 3-way to a 2-way for NS1000M. My concern is that part of the crossover circuit may still be in operation and I am hesitant about cutting out parts without guidance...so back to the books