Not sure I'm understanding your question right, but I did used to have an EV controller with my first DS50 on a point-n-shoot, before I got my Ike housing with the iTTL built in.
The EV does optical or hotshoe-based syncing and lets you set strobe power (the strobe head itself should be set to full pwr, TTL). So if shooting manual, and you can use the internal D300 flash to trigger, then I'd say "yes" it will let you set power on a SINGLE strobe. The EV controller only has one 'output' signal with the pigtailed connector: although you can hook up a dual sync cord to it, that's for "input" (e.g. from camera to feed two EV controllers).
So if you currently are using a dual sync cord, I guess that means you have an adaptor to your Subal housing providing the iTTL signal to your DS125s, in which case you'd then need not one but TWO EV controllers, each of which would pigtail to an individual strobe, so you can set power on each one via the controller. But if you can use the internal flash on the DS300, you could just use the optical triggers in the EV controller(s) and ditch the dual sync cord entirely, so you'd only have the pigtail connections between the strobe heads and the EV controllers.
At least that's my understanding of the system. The manual for the EV is here: http://www.ikelite.com/downloads/instructi...-Controller.pdf