Firstly, if you wanted to keep roughly the same stylings, the Shift2 is out next month I believe.
Null - decent barrel, just need to make sure most of your paint is around the 685-691 mark.
Lotus - rather nice, probably my favourite 14", no CF barrel.
CP - unless your being cheap (as to which, its a great barrel), I wouldn't bother.
Disruptive - do you like 15" barrels, if so, easily one of the best barrels on the market.
Un1tec - can't comment on it, looks to have a tonne of indented porting, which should keep it nice and quiet, with a minimum of bunker rub getting it.
If your trying to match the 'style' of your RX though, a SHIFT2 or a Lotus would be your best bet.