It's funny about the AC701. I have about 8 mics, Neumann and Schoeps, that use the AC701 and AC701k. I've never had an issue with a tube going out.
Why? If you read posts from Klaus and Oliver, they will tell you that the key is calibrating +H voltage to deliver somewhere between 3.85-3.95V (but never over 4.00 V!).
I calibrate my PSUs every 2-3 months, and don't have the slightest sign of a tube going out. So, not sure that replacement AC701s would be needed, at least for those who practice 'religious' PSU calibration.