I haven't done it myself, but I know you can replace the bulbs with LED's. Here in Chicago Deltronics does it, so check with your local competent repair shop. The LEDs will last a lifetime.
I have owned a number of various Mc amps, pre-amps and had to replace bulbs in some of the earlier models. But only due to burnout of the bulb. The ceramic/porcelain base was always in tact, this is first time I have ever even heard of this issue. I am surprised that Audio Classic was unable to assist you. Did you ask Audio Classics if they had an LED retrofit kit available? They make them other older Mc gear, I would ask them again. Sorry I can't be of more help.