Know the model number? Looked for any product information/user manuals from Rainbow's site?
Although the range is listed from 55Hz to 6000Hz, 55Hz generally being the resonance frequency, Fs, the usable upper end normally not so usable. If I were to guess, you'd be looking at 2000Hz to 3000Hz tops for a normal 6.5" woofer. The 2000Hz-3000Hz x-over range is very common for most 6.5" woofers and a good bet for a starting point. If you're running active, you are free to play around with x-over points and slopes and find the best setup. If you plan on buying/building a passive x-over for it, then you kind of have to shoot for a safe point and hope all turns out well, lol, not exactly the best route. There are a lot of relatively affordalbe high end HUs capable of controlling an active setup, Pioneer's DEH-P880PRS being a good one at under $300. There are also plenty of basic to extensive seperate x-over processing components available as well. Heck, even some amps do have full x-over control to handle a 2-way + sub setup, although, they're usually not cheap.
You didn't really say what you had for hardware, so I'm just sort of providing some suggestions. If all you needed was a good starting point, 2000Hz to 3000Hz should be your sweet spot. It just sort of depends on how high the woofer can play before introducing distortion/breakup/beaming and how low the tweeter can play before you start over excurting it.