We picked up $8,000 worth of Tube Traps for $300.00 after a local used equipment dealer put them up for sale as "display pedestals". Best money I ever spent. Art Noxon is genius IMHO.
Having had 27 traps in my listening room, that would be enough traps to treat a room quite well imo. The beauty of tube traps is that they are--by nature--more broadband than nearly all other devices that remove energy from the room; because of this, they are more 'acoustic problem solvers' that 'physical low pass tone controls' (like most other panel-type/wall treatment absorbers).