18sound 8nm610 mod / recone?

I acquired a pair of these beauties for a reasonable price, mainly because of their very high efficiency. I would, however, like to lower the rolloff (fs in the high 200s with roll-off only a tad below 1 kHz due to very low Q) by opening this sealed chamber.

They are self-enlosed, i.e. the basket is more like a 2.5 l bucket. From the pics, I thought it was screwed onto a conventional basket, but it really forms one piece with the flange. There are two sets of screws on the back, one I suppose holds the connector terminal, the other probably holds the motor in this monster bucket/basket.

Has anybody ever had to recone these things? I can't believe they cast the flange, bucket and whatever holds the spider in place in one piece, so I suspect there is a gap hiding underneath the gasket but I'd hate to ruin gasket and possibly surround in the process of finding out.