I thought I read somewhere that as frequencies get lower, the standing waves inside a long tube create problems with resonance, pressure on the back of the driver, and cone excursion problems, which can cause smearing, congestion, and a tubey (not vacuum tubey) sound.
John, I don't know the answer to your qestion about felt, but felt might be a form of wool product. I noticed at the fabric store that there was more than one kind of felt and in different gauges.