Eminence has gone to some pretty impressive lengths in the past few years, to sell speakers- you might want to check that out. Their website has sound files for all the speakers they sell- best to use that as a comparision from one E-speaker to the next, but less useful to compare them to other brands. They have also distributed to retailers, a 4X12 cab with 4 different speakers in it, and switching that allows you to compare the four, with your rig. Yeah, I know that doesn't call for rocket science, but it's a good resource.
In general, speakers are one area where I am usually happy to go with new, or nearly new. Vintage repro's don't seem to be pull-your-eyes-out expensive, yet appear ("a-hear?") to be faitful reproductions of the oldies.
That said, I seldom hear a huge difference when replacing speakers in guitar cabs. I have done it at least three times, recently- here is what I did, what I did it on, and how it sounded:
1. Replaced the original ALNECO 6-incher in a Monkey-Wards Model 6000 guitar amp with a 6" Weber Blue Pup. Amp is a "radio-tube" 60's guitar amp with tremolo. Scary as all hell to play because it does not have an input transformer- that being the design that kills guitarist. Amp can not play clean at any volume that does you any good, I replaced the speaker because I thought a replacement might clean up the sound a bit, and because I scored the Blue Pup at a garage sale for something stupid like seventy-five cents. It did help some, mostly because it seems to be a more efficient speaker, allowing the amp to return more volume at a lower volume setting. Mostly, a demonstration of the "You can't make a silk purse from a sow's ear" truism. The blue frame of the new speaker really looks cool, sitting inside that open-back cab, though.
2. Replaced original Fender "Special Design" 10 in a Super Champ XD with a Eminence Ragin Cajun. The SCXD is light on bass; this swap is widly regarded as the best "fix," and the #1 mod for that amp. I remain a tad underwhelmed- better bass response, but not a whole truck load of the stuff. Nowadays, Eminence has another 10 that has lower bass response and bass efficiency- if I were doing it now, I'd go with that one. And the "Eminence Patriot Series Ragin Cajun" sticker on the back of the speaker looks cool, sitting inside that open-back cab...
3. Removing a Celestion 12 from a Peavey Bandit 65, and replacing it with a decent but not particularly impressive 12 I had around here. (Celestion want back in the Red Bear 412 it came out of, to be among it's bretheran.) Bandit 65 was in rock n roll summer camp rental use, so the Celeston was wasted there. No big difference in tone- sort of a demonstration of the "weak link" phenomenon.