My 4x12 is loaded with a pair of older Celestion G12-75's (no T) and a pair of JBL K-120's (100 watt Alnicos). Love it. I also use this cabinet with my Express, and love it for that too. I guess I just love this cab 
 
  
  I can't wait to hear it with the blonde Bassman I am working on.
But I digress....
Rockets love to be bright and chimey, so an overly dark speaker is not the best match., Also, they have nice tight bottom end, so a speaker that gives up too soon on the bottom would be a shame too. G12H30's are probably very nice (mine is in a 1x12 so I haven't tried it), and most Alnico's should really bring it too.