I've put up a few antennas in Manassas & have found channel 9 to be one of the easiest DC channels to get. I just ran my stepmothers TV FOOL & the signal strengths are similar, so you should have adequate signal. Some type of electrical interference may be hurting you as Tower Guy mentioned so you may need to investigate that as a first step.
Have you tried removing the amp completely? If so, you may need a different antenna. If you don't care about channel 50, the RCA ANT-751 has worked quite well. If you need channel 50, you will need at least a Winegard 7695 or better to recieve their weak signal. I have an ANT-751 you can try if you would like to test before buying one.
You can also augment the 4228 with a
small VHF antenna as Dave Loudin suggested for a little less money. You would join the 2 antennas together with a
UVSJ.
You shouldn't need an amp with just 1 TV connected.