More searching 40mm drivers has revealed some more options.
JVC XX Extreme Bass. I think this combination might be dangerous. Think of Beats meets Grado. Granted, it's a closed can, but with a bass port. Not sure how it would behave in a wooden cup, though. The specs indicate it is capable down to 7Hz. They only cost about $29.00 a pair.
Or, JVC HAS400B. This is the closed headphone from JVC based on the Nanotube technology. I've had the HAS400B as well as the HAS500B before. The 400 has the 40mm drivers. The 500 has the 50mm drivers. Some say the upper frequencies are tipped up a bit on the 500. I remember more about the 500 than I do the 400 as the 400 I only had for a few days. I recall that the 500 had great bass as well as upper frequencies. The mids seemed a bit lacking. But, this might have been from the 50mm driver and the additional emphasis on upper frequencies. Though, for the money (about $50 for the 500), I was impressed with what these were capable of. What did I not like? The fit. The headphones had a tight fit and had the capability to squeeze your head quite hard. For those who complain about the Grados not being comfortable, they'd really hate the JVC in its form unless they had a very small head. The 400s run $39.99 and are local for me.
I think with the way the plastic baffle is designed, one could cut down the baffle with a Dremel and have it fall in the 45mm range for a better fit inside a wooden Grado cup. I'm just not sure what would happen to the sound, though. The rear of the drivers has similarities to others with the mesh on the back and some form of holes on the rear.
If I can get over to the store tomorrow, I might proceed with a purchase of these and give them a spin. While their price falls between the Koss clip-ons and the PX100 II, I think the driver might be quite a bit better than that of the Koss, but still not sure about using it in an open cup design.
I should also jump on some of those $3.00 wonders from China. I'm just not sure If I'm patient enough to wait 2 to 3 weeks. But, on the other hand, I could then have a stack of projects to try with the Grado cups over the winter. Since the 4 wooden cup samples are heading my way, I'll most likely use either the Sennheiser driver or possibly, the JVC HAS400b driver in the testing. I'll have to give these some thought. I've had the XX Extreme before, too. It was just an odd splurge. I don't remember too much about it other than the major clamping force on my head and quickly suffered headaches. But, the bass on that unit seems a bit too far out there for serious consideration.