I managed to get KXTU to work by taking all of the splitters out of the line and running just a straight wire to one tuner. This gets me 23 dB SNR, which is adequate. All other locals, even split 4 ways, are better than 30dB. At least I know there is still some kind of signal; now I just need to decide how best to use it.
So, I'm thinking about using a distribution amplifier (CM-3414, perhaps) instead of splitters (currently a 1->2 with another 1->2 on each branch). Anyone else tried this?