Well, if it makes you feel any better, my FIL has an HBU-22 aimed right into a 25 ft high wall of coniferous hedge that you can't see anything, even daylight, through with a fool report very similar to yours, but his is mounted outside. He gets flawless reception.
Doesn't mean it'll work for you, of course, especially in the attic, but if it were me, I'd get something like the HBU-55 (high gain, tight pattern, UHF plus H-VHF) and try it. Since all your towers are in a tight pattern, you might get lucky, though you might have to mount it outside in the end.
Or not. It's easy to spend my hypothetical money, since it doesn't actually cost me any real money.
