OK, so I determined that an amplifier is useless, unless the RCA I bought at Lowe's is a piece of junk, which I wouldn't rule out given the brand. Earlier today I connected this really old Sony HD receiver that has tuners for OTA digital and analog as well as Directv, mostly because it has a strength meter that seems more reliable than the one in Windows Media Center. This one has like 25 bars and two numbers, a real time and a peak number, but I don't know if the number is simply a percentage or something else, but it seems to be a percentage. I connected the antenna as it is, and I got these peaks:
Channel 11 (11.1) = 0
Channel 17 (17.1) = 74
Channel 48 (5.1) = 76
Channel 49 (50.1) = 74
However, with the amplifier, the peaks barely changed: 0, 77, 79 and 77, and these are not very scientific tests, it's just after ten seconds or so for each channel.
Funny thing is, I have this old Radio Shack indoor antenna that is really fancy and even has a remote control, so I put it on top of the PC and WMC gets channel 11 without a hitch, and the others as well. However, when I connected it to the Sony receiver, the level for Ch 11 was still 0, and for the other channels it was lower than with the attic antenna.
So my next step will be to check on the attic antenna, replace the connector, and try to mount it in a way that is directly facing the towers, because now it's at a small angle. If that doesn't work, I can install the AverMedia tuner again, and have two tuners, one for Ch 11 and the other for the other channels.