I tried doing a search, but I couldn't come up with anything super conclusive. I've seen a few people say they don't like regulated heater supplies in tube mics, especially U67s, but I can't find a reason why. The heater is negative in a U67, so a 337 needs to be used instead of a 317...no problem there. Regulators can introduce some noise, but regulating a volt or so below -6.3V (i.e. -7.5V) and then doing some additional RC filtering should solve that. It can't be an issue of current supply, since we're talking about the heater, not B+, and besides, it's easy to build a circuit that can supply plenty of current. So....what's the issue?