Try to make sure that whoever you have work on them is very familiar with these regs. Lots of MK15s have been destroyed by well-meaning techs trying to dig that bushing out. It looks very similar to the replaceable bushing in the MK20/25, but it's permanently installed in the MK15. Mention that to the guy and see what he says. If you get a blank stare, consider going somewhere else.
I'm not sure what you mean by "same with the 1st stage" free-flowing, about the only thing I can think of would be a leak at the HP o-ring that would cause a slight leak out the ambient chamber holes. Is that what you mean?
The D400 has a very light spring pressure on a relatively large seat surface, so developing free-flows in storage is pretty unusual. I've got 6 of them (actually D300s) and none have ever done that. What does happen, fairly frequently I suspect, is that the spring gets a bit torqued in the adjustment process, then relaxes a bit with some storage or use. It's really easy to fix, it just needs to be someone that knows the regs. Surprisingly few techs are familiar with these fantastic 2nd stages, and I bet that was a big reason SP discontinued them.
I wasn't trying to worm your reg out of you, I have plenty of them. Too many, really......