Hey Haryn. Well get everything up and running first, then worry about the tube amp. But I do recommend it for the horns.

Just pop me an email for the cables. I don't sell cables, but would be happy to build a set for you. They'll just be RCA to XLR to get you into and out of the Rane crossover. I use Canare, Belden or Mogami cable. Great pro stuff.
Once you get the crossover, let me know. Then you'll need to figure out how long the cables have to be to reach everything.