I've got 2 G12M Greenbacks from 1976 and they sound just as you describe, not a lot of low end or highs, kinda middy but in a pleasing way.
I tried several speakers with them, V30, some Jensens, some others, and then I got these Celestion G12L speakers that are 30W...I don't know the exact model but they are made for some Ampegs, have Celestion/Ampeg logos and are very small magnet. I really dig the way those pairs work together, I get a rich focused low end that is so big I seem so sometimes shadow our bassist when I do some chunca chunca on low strings - but the frequencies aren't bass-low anyway. They also have a completely different high end response which makes the cab sound like there is a superb reverb actually, very 'stereo' somehow. I just got back from the training and once again I was so glad about the sound, it's simply just right.
But, I don't know...Celestion doesn't seem to have anything looking like that on their webpage.
These seem a lot like mine:
http://www.google.fi/imgres?imgurl=h...26tbs%3Disch:1
Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.