Even if the Eminence are in 12", don't think they would be a great improvement over the tone of the currentt speaker. Very few people supply speakers here in 12" size as there is little market in replacing them.
Try changing the preamp tubes over to different brands (the first one from the right when vieving from the rear is the first preamp stage and has a great influence on tone).
I have Clestion Greenbacks which would work, but are 16 ohm and I believe your amp is rated for an eight ohm speaker, so that's a no go. Otherwise consider bringing in a Weber from the US. but this will cost at lkeast USD150 for the speaker and shipping. Better than the current Celestions.
cheers,
Marki.