Greetings ETMegabyte,
Welcome to the mystical magical land of over the air (OTA) reception! This plot is only the beginning as there are dozens of factors at your location that can work for you, or, work against you.
In regards to your indoor amplified TV antenna. Never rely upon those as a barometer of the best reception possible at your location. They will often only pick up the strongest of the strong. Remember, TV signals have to penetrate walls and as they penetrate walls they weaken substantially. It is no surprise to me you see ION and the PBS as they have barn burning signals. It is also no surprise ABC doesn't decode. You are in 2 -edge conditions (2 obstruction + your walls) along with a nearly 25 db less signal. In short, ABC will likely require an outdoor antenna.
This is where the bad news kicks in. While ION, PBS and ABC are very do-able with a moderate outdoor antenna, the other "big" 5 have considerable difficulties. NBC, FOX and CBS are in very poor signal. Additionally you have the misfortune of affiliates being spread out over several headings. This puts you into a very large antenna to even have a chance at all. Add to that, if you are in an area that is surrounded by trees it becomes even less likely for reception success of NBC, FOX and CBS as trees reduce signal strength greatly due to reflection.
Signals once they start to hit about +10db signal strength (red shaded), they become prone to atmospheric variances. The result is that you may see them some portions of the day, while other times they may drop out. Boston is in this category, however it's more severe as many are around 0db or negative signal strength. This adds up to challenges.
At the end of the day though, Boston yields you the best chance at CBS, FOX and NBC due to all being at one heading. An antenna will be able to focus all its reception power in one specific direction. If I were to attempt Boston I would purchase an
Antennacraft Super-G 1483. It can be found
here. This is a deep fringe unit, for use in extreme fringe like you are located. Orientate antenna to magnetic heading 190.
Your plot doesn't have a height, however, I'd make sure to get the antenna mounted outdoors above the treeline (if present).
I would expect your local ION off the back. ABC *may* decode. With UHF designs, many times they can receive stronger VHF signals if present. If ABC doesn't decode you may need to add a small supplemental high-VHF for ABC reception.
This isn't a slam dunk. Far from it. If you embark on this attempt, I'd put your chances at Boston in optimum settings at 50/50.
In the world of OTA TV, sometimes you just don't know until you put something in the air and try.
RE: how many TVs can you hook up. There are currently distribution amplifiers that work with OTA signals that allow you to split your antenna feed to eight TVs. A mast mounted preamplifier you are not a candidate for with the presence of 70 db signals nearby.
Best of luck.