Not an antenna engineer, or part of the industry, but I am an engineer.
12 and 8 should be fine with a Y5713 pointed at them, no amplifier, assuming there are no trees or buildings blocking the horizon. Then pick how much you want to spend on the UHF antenna. An Antennas Direct DB4 would probably do it, pointed at 281. 15 is considerably down from 17. You could buy a cheap UHF antenna like this one
http://www.ebay.com/itm/Eagle-Aspen-...62798993&rt=nc and try it out.
You can buy combined antennas like these
http://www.antennacraft.net/Antennas/AntennasHBU.html but I think you may need the extra sensitivity of a dedicated UHF antenna to pull in 15 reliably.