Where do you live?
Remember... all HD stations are digital, but not all digital signals are HD. I'm minutes outside of NYC and we only get 7 HD channels out of about 40 digital channels. but I will say that a non-HD digital picture can be prety durn good.
What antenna do you have? Some already have an amp built in.
It's just a VHF/UHF (and possibly FM) amp and unless you can mount it at the antenna itself, not inside, any improvement will be minimal.
Remember, amplifiers amplify the "bad" noise just as well as the intended "good" signal so the the ultimate result might be a strong, noisy signal instead ofa weak one.
I'm in position where I can catch signals from Charlotte, Winston-Salem, High Point, Greensboro & Raleigh, NC. So, I have a couple of Fox, ABC, CBS, etc.
I'll have to count how many are 720 or better. But, you're right about the SD signals, they're much better than my cable signal.
The antenna is a Phillips MANT-950. It has 15db/18db amplification built in for VHF/UHF, not sure if it's in that order though.
Right now, it is mounted on an old mini-dish mount. I'm going to pick up some antenna post today to raise it since it is currently below the peak of the roof. I think 2 sections of antenna post will get it over the crest of a nearby ridge and reduce any interference from the trees.