It has 'tighter' more controlled lows, meaning it doesn't extend as deep as ported cabs, but has plenty of punch. Think more classic bass tone, like the sealed Ampeg 8x10. Still lots of energy down low where it counts in the 60Hz region! There is no tweeter so it has a nice smooth top end, which sounds GREAT with fuzz and distortion feeding it.
Also, the drivers in sealed cabs are usually more consistent in their impedance rating throughout the frequency range, whereas the drivers used in ported cabs tend to have a big peak in impedance at a certain frequency. Valve amps are designed to work best with a constant impedance load, hence valve heads usually being 'happier' when hooked up to sealed cabs. Solid state and hybrid amps aren't so fussy and work well with both types of cab.