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I'm on the good trader list with a X12.

Up for sale is a Celestion G12K-85 8 ohm. I'll take $45 plus shipping, which will be around $20.
I'd be willing to trade for an 8 ohm Vintage 30 or a V30 clone, like a WGS Veteran 30 or one of the others that are out there.
It came out of an old Crate G80XL amp that I had. I think it was made in the 80's. It was made in England. It's thoughly broken in and sounds great. It just produces too much bass and doesn't sound good with my Mesa. It has sounded great with every other amp I've used it in though.

Celestion changed the name of this speaker from G12K-85 to G12K-100, but they are the exact same speaker. They just decided it could handle 100 watts and changed the name.

Celestion’s Description:
Celestion’s very own monster of rock, the G12K-100, makes full use of the heaviest G12 magnet to combine huge power handling with superb clarity across a wide frequency response.

A massive bottom-end, rock-hard midrange and restrained top-end make this the perfect speaker to add thump and grind to modern high-gain tones.

Loaded in 4x12s it delivers its famous sledgehammer lows, as favoured by Slayer’s Kerry King, but it also often finds use in small boutique combos, bringing a powerful mix of low-end warmth and mid-range attack.

General specifications
Nominal diameter 12", 305mm
Power Rating 100W
Nominal impedance 8Ω
Sensitivity 99dB
Chassis type Pressed steel
Voice coil diameter 1.75", 44.5mm
Voice coil material Round copper
Magnet type Ceramic
Magnet weight 50oz, 1.42kg
Frequency range 80-5500Hz
Resonance frequency, Fs 85Hz
DC resistance, Re 7Ω
Mounting Information
Diameter 12.2", 309mm
Overall depth 5.3", 135mm
Magnet structure diameter 6.1", 156mm
Cut-out diameter 11.1", 283mm
Mounting slot dimensions 0.31", 7.9mm Ø
Number of mounting slots 4
Mounting slot PCD 11.7", 297mm
Unit weight 10.4lb, 4.7kg

Now for the pics: It's a little dusty, but it's great shape other than that.

If you have any questions or want more pics, PM me or post in this thread.
I don't know man. I'm kinda looking for stuff to change up/ add to my tone. I've got my b-52 at112, which is basically a Mesa's little cousin. So, since you said it didn't sound good in your Mesa, I'm reluctant...
It didn't really sound bad with my Mesa. It just made it sound too bassy for my taste. Since it's already used you could try it and if you don't like it re-sell it and get your money back or trade it for a different speaker. It's cheap enough that it's worth a try. You might like it.