That's a tuff spot. There are later thread on's but don't know how low and if the compensation curve will match the driver. Can't believe how expensive they are! You could do aftermarket diaphragms to get started have you looked for OEM replacements on Ebay?
If you can, find a cheap pair or 2425S's or 2426's as an upgrade that will work. You can get JBL replacement diaphragm's both 8 and 16
Rob