hey mate, it only appears to have 1 rca preout for a sub, it also has an av input and an av output (for say xbox, and also an out to display to an external source).. i am unsure of whether you can use the av in as a sub channel, you might have to ask experts on that..
and as for a separate sub control, i havent looked into that as of yet, i think it may only be a rear/front bass treble metre, i havent explored the rest of the options, but i'll ask my mate to check the manual a bit more..
by the way, nice car, gotta love the r34's
Rick