I just converted my new Origin 20c Combo to a 12" Celestion 30 Anniversary. No matter how hard I hoped, I could not find a 10" speaker that sounded good to me. So I had to do the Mod. For anyone considering it: First Disassemble the amp & remove the Grill Cloth & Gold piping Cut an 11" hole for the new 12" Celestion and Countersink the speaker mounting threads New Celestion Fits snug, but fits Amp Re-assembles back to stock, everything fits. Modification Undetectable, amp sounds awesome!