The problem is, you have discounted the possibility of there being a problem in the loft without actually checking. As you still have a problem and nothing else you have tried has fixed it, you need to stop discounting the remaining possibilities simply because you don't see how they might apply.
If you can't lay your hands directly on the problem you're going to have to go right back to basics, by connecting one TV from upstairs (because you know that those TVs actually work) directly to the cable that comes down from the aerial. If/when you establish that there is a good signal coming from the aerial you can then put everything back one piece at a time until you discover which part is breaking everything else.
Based on what you have said so far, my own suspicion is similar to that posted above by nodrogd - there is still a broken amp in your set-up somewhere. As you have had a faulty amp behind the TV as well, it is possible that it has power surged as it has failed, and passed on the surge, damaging other equipment in the process.
However, whether this is the answer, or something else, you are only going to discover the problem by a patient and thorough process of elimination - and that doesn't include failing to check things because you don't see how there could be anything wrong with them.
So, go and get in the loft.
