The coffee our old Mr. Coffee (on the counter, next to Mr. Radar) would brew used to vary unpredictably in quality. Same beans, same quantity, same water — one day’s batch was great, the next: Pfft!
I caught a clue from a PBS cooking show episode that tested coffee makers. The key factor in producing good brew was the temperature of the water going into the grounds. The problem all the lower-rated coffee makers shared was that the water lost too much energy between the boiler and the grounds basket. Another shared factor was that the path from the boiler to the basket was through a tube that passed through the water reservoir — just like my erratic Mr. Coffee.
Hmmm. The bad brew days were, as I recall, on really cold mornings when I especially needed a good cup to get started. The water going into the tank was just as cold as what was coming out of the faucet (a reverse-osmosis filter rig). Energy transfer through that tube between the boiler and basket would be proportional to the difference in temperature between the inside and outside of the tube, so filling the reservoir with warm water should give better, more predictable results.
It works. A microwave oven and a large Pyrex measuring pitcher get the process started. Just remember that you want warm water in the tank, not hot.