The Line 6 Celestion G12H-90 speakers fitted in my DT25 112 extension cabs were excellent, but they're not a patch on the KEMPER Kone. Because of the superior frequency response of the Kone, you can really hear the difference between the various speaker imprints. The difference in sound is on another level, they seem to reproduce every nuance of the profiled amp. That lovely creamy valve amp tone just comes shining through; they're well worth the extra money.
Have you thought about putting a pair of small ports in the front of your cab, it let the speaker "breathe", or is it open-back cab? Also, has it got baffling on the inside? The sound out of the Line 6 cabs is huge, the bass response is amazing for the size. The closed-back, front-ported design, and the baffling, really seems to make a big difference.