I'm not familiar with the X650 but I'm pretty sure it is the same as my S600. First you have to wrestle that damn hose end cover off. I'm not sure how it is supposed to be done but, as you seem to have already found, enough tugging and twisting and it comes free. Then you should find 2 knurled nuts which scubapro so thoughtfully uses rather than std hex nuts so you can't tamper with what you own. I use soft jaw pliers, holding the nut next to the case steady and unscrewing the one on the end of the hose. Scubapro has a special tool for doing this - also for your own good. Regular pliers with something to protect the finish can also be used. Trick is to find one thin enough to grab that inner nut without overlapping the outer nut.
BTW, why are you trying to disconnect that hose? Any std hose will reconnect. Just make sure you hold the inner nut as you do not want that one to turn in relation to the case. It is snuggly holding the air barrel in place. Too tight may distort or crack the case. Too loose may leak under vacuum.