Doubling the wattage gives 3db of increased sound level, not so much in absolute terms. However I'm starting to think cabling. I would start by replacing the speaker wire with whatever's around the house, or buy some 16 ga zip, quality doesn't matter right now. Then replace the interconnects, again, quality doesn't matter here. You're looking to find a break in the wire, or in one of the connectors. Also, do have a receiver or cheap amp around to troubleshoot as well, something to stand in for the TBI? At my house there's 5 or 6 amps and dozens of cables, I have to remember others haven't been collecting junk as long as I have.