For now the problem has been fixed: I talked by chat with AT&T after my posting. The tech determined there was indeed a problem on the line on the AT&T side. This morning download speed was back up to normal (for my neighborhood's modest DSL speed, roughly 2 megs) and I got a call from the AT&T field tech confirming that the line problem had been fixed. Unfortunately he did not explain to me what, exactly, the line problem had been. I'm keeping my fingers crossed.
I had connected the DSL and phone cables to their respective jacks on the old filter (which I had been using without any problem for several years) and then on the new filter. The telling thing was that initially there was no speed problem when I swapped out the old filter; it was only three days later that download speed crashed again. In all fairness, I am perfectly capable of connecting things bass-ackwards, although my idiocy turned out not to be a factor in this situation.