I have 3/8" thick, 10 lbs density underpad with high quality berber. It's 15 yrs old and still looks brand new even though I've dragged speakers and subs all over it. Back in 2007, I paid 5 grand Can for stairs and 680 sq ft for removal and disposal of the old carpet and underpad, and installation and material of the new. It's warm and sounds fruity compared to the nutty sound of the old carpet.
Don't knock the furnace filters and stuffing until you try it. You may want to include a couple of in-line, galvanized aluminum humidifier evaporator pads halfway in between the furnace filters and the wall. I found improved separation of details with the M100s. With the actives, I don't need the pads.
