I'm not trying to speak for Bill, as he is the de-facto expert on Grado-woodying, but I currently have my SR60's on, listening to International Superhits by Greenday. My pair have the basic rear ring done in oak, leaving the front ring on the driver to remain plastic. I wouldn't say they amplify the bass so much as they improve it. I feel the bass has a little bit more crispness to it, possibly because the oak has a fairly high acoustic velocity. The hole-punch mod brings out the quantity of bass, but the wood rings define it. They also feel to make the sound more "musical", and I wish I had a better way to describe that quality. I noticed it the most on the live acoustic track of "Broken" by Seether. Being a (very) amature guitar player, the sound that the headphones reproduced felt very much like I was playing or hearing a real acoustic guitar. The wood shells seem to give a more satisfactory resonance to the sound (after all, they don't usually make acoustic guitars out of plastic, do they?) I'm having a hard time putting it into words but the guitar notes "ring" better. Not longer but in a more appealing way. Perhaps it has something to do with the harmonics of the wood? I have no clue, but unlike the plastic ones, individual pickstrokes seem to reverberate down my spine like they would if I was actually playing. I can only imagine how the 2 stage wood sounds, and I think my new goal in life is to one day get an SR325 and do the front chamber in wood.
I hope this helps instead of making things more confusing.