Best furniture to act as bass trap

Based on what Erik stated, a leather couch may be a better bass trap.  The harder leather surface may act more as a "membrane" -- this is typically used in bass traps as a more effective way of absorbing lower frequencies and at the same time reflecting higher frequencies.  Look at specs for 703 FRK fiberglass panels (that use a foil type membrane on the front).

I think the above poster is thinking of a Tuned Membrane Bass Trap.  This is, of course, tuned.  The couch is not.

@rand-11  - yeah, totally understand.  It's not a tuned basstrap.  However, the 703 FRK is not a tuned bass trap either.  I'm just trying to say that there's some element of more efficient bass absorption when you place a flexible reflective material in front of a highly absorbive material (i.e. the 703 FRK).  It's not going to be perfect and it's not going to be a "tuned bass trap", but I would error on the side that there may be some benefit to the leather couch over the cloth couch. :)

The membrane better converts the longer wavelengths to heat energy than does the absorptive material. 

Toilets make a great bass trap. They are customizable too - depending where you sit and/or the fluid/solid levels you can tune them for the best sound. The only drawback is fitting the rest of your gear in the toilet - usually the door won’t close - audio is frustrating.

Seriously for good bass trap you need at least three if not four GIK corner traps. A large smooth leather sofa will be best. Cloth will absorb too much highs relative to lows.