If you want to replace your internal passive crossover with an external crossover, then you need separate wires to your tweeters, midrange, and woofer from the crossover.
I looked up the Mc901. It is a hybrid amp that wants to drive your woofers with solid state and your mids and tweeter with tube. To do this your speakers have to be set up for biwiring.
If you speakers are set up for biwiring then you could remove the internal crossover from your woofer if you want. I don't think this is the intention, but it probably would be good to have the amp crossover set about the same frequency as the internal crossover.
You still need to upper crossover to separate the mid frequencies from the tweeters.
If your speakers aren't set up for bi-amping, I don't know if the mc901 will accommodate that or not.
Jerry