Sorry for the length of these, Paul, it's amazing how long something can get with this kind of stuff.
Boy, that's a bummer you can't pull in WCPO-DT consistantly. It certianly sounds like your VHF reception is working very well. Since you are so close to Dayton Towers and probably have to point right through them from your location to aim at Cincy, just about any preamp would probably cause more problems interference wise than it solves gain wise, because Dayton analogs 2,7,16,26,45 are probably causing intermod and overloading the amp which is probably scattering noise and everything else onto channels which should be clean.
There are "usually fairly expensive" ways of trapping out those strong signals so you can get good results with a preamp(although you wouldn't be able to see them then with the specific antenna being used), but it's probably not worth all the fuss, especially since in the future we don't really know yet which channels the local DTV's will end up using.
The only other thing I could guess of besides poor signal strength(which certianly seems to be the problem you're having with WCPO-DT) would be that you are getting some sort of weird interference to 10 -- or even though you can't see or hear it, WBNS signal may be raising the noise level enough most of the time to prevent good reception of WCPO-DT.
I can't imagine that it would be possible to get overload at the tuner from a Dayton local that would "scatter" it's signal onto 10, since the closest frequency used would be ch. 7. Since your getting good reception from 9, this certianly shouldn't be an issue. Luckily, tuner overload onto 1st adjacent channels to Dayton 2,7,16,26,45(12-13 Miles) and WKOI 43 (TBN) Richmond, IN(12 Miles - tower is actually between Trenton and Oxford, OH) is my only real issue here, and it only happens when I have the antenna headed near those stations. For 2,7,16+22, it actually happens to some extent without a preamp, but the preamp makes it worse. This isn't a problem though, as from my location, the preamp does happen to do more good than harm.
Harmonics, or Spurious transmission from a very close by high power FM transmitter shouldn't be an issue either as the 1st harmonic of an FM transmitter that would affect Ch 10 would have to be on 96~98.5 MHZ, and there aren't any on those frequencies in Dayton. 99.1 could really affect only Channel 11, and you would have to be awfully close to their tower, with a very high gain setup.
I'm sure you've thought of this, but if that 19" TV is of the non-screen blanking type, tuning it to 10 and seeing if there is anything there besides normal, clean snow+ "white" noise audio can give you an indication of interference present.
BTW, along with 4+6 Columbus(76 Miles), I get WAVE here all the time to some extent(1/2 the time or so it's watchable, but the audio is fine 99% of the time) if antenna is headed correctly, at 95 Miles out, I'm not actually that far away from the edge of WAVE's predicted coverage area, which reaches out 72 miles "curvature of earth" wise because they have an extremely tall tower that is about 500 feet higher elevation wise than most of the stations in this area. Also, thier low VHF frequency lets it "bend" around the earth farther.
Once WDTN gets on the air(fingers crossed for Nov 1), I hope it doesn't take too long for them to pass all the ABC HD!