I have 2 Marshall JCM 800 era 1960 cabinets that originally had the horrid G12M-75s (mid scoop, way too much low end, nothing nice about them). I upgraded my slant cabinet (the only one I actually use) with Classic Leads just on faith from what other users told me and a couple of sound clips (V30s were my other option). I will never use the straight cabinet with the G12M75s in it again after the change.
They sound so much better than most other speakers out there that they'll still sound better than most new while you wait for them to break in.
In a nutshell, they are like V30s with more bass (a lot more but that's not saying much, but less than G12M75s), and less highs but the same great smooth mids that makes the V30s. One thing about them is you need a good amp.... if there's any flaws in the amp, the Classic Leads wont make them ugly. And you'll also want to down your preamp gain with these and up your master a bit too because your amp will sound a bit fizzier but they get warm when you start pushing power tubes so your amp sounds warmer.
They also work really well clean. I highly recommend them for Marshall style amps, but stick with V30s for Mesas or if you want an older style tone. The other speakers I can recommend is the G12H30s but they will break up, but they're loads better than Greenbacks.
Celestion has samples here, I think the same holds true...
http://professional.celestion.com/guitar/features/tone/index.aspThey're my fav. speaks personally. I did some clips with my Rebel Yell loaded Les Paul through my modified Marshall JCM 900 SL-X but unfortunately I did most of them with the straight cabinet... what happened was I wanted to see how that thing sounded with the G12M75s... terrible, as I suspected, but I decided to record at the same moment.
It should be noted that on the G12M75 clips, the mid was at 10, and only at 5-7 on the Classic Lead clips.
http://bareknucklepickups.co.uk/forum/viewtopic.php?t=4562