I already own 6 D400's myself and would always be glad to buy another if the price is reasonable. PM me before you decide to run over it with your truck.
Following any air sharing drill, I can rely on getting comments from the assigned dive buddy about how great one of my D400s breathes compared to his reg regardless of what brand or model of reg he happens to have. They are extremely easy breathing regs and achieve excellent performance without the unnatural over inflating feeling that many high perfromance second stages have.
In my experience Scubapro seems to have a problem with caving into the demands of whiney techs who don't like to work on "different" regs. The D400 is different to service but is not hard to service. Some confusion arises as some techs don't know the difference between a D300, D350 or D400 or that there are differences in terms of the levers and how they are installed. I can rely on having one a year come through the shop for annual service where I find the lever installed backwards.
The adjustment procedure is substantially different than with a more conventional single adjustment, dual adjustement or balanced second stage design but again is not hard to perform if the tech can read at a 6th grade level as the manual spells out the specific procedures and sequence.
One of the bigger mistakes is in adjusting the spring tension on the poppet where the tech does not turn the spring adjustment pad past the desired position and then back, which consequently "winds" the spring a bit, and pretty much guarentees the poppet will leak a dive or two later. The other common mistake is to take the slack out of the lever before completing the adjustment of the poppet.
The late production D400's (post 1996 or so) used a plastic orifice (due to complaints from whiney techs about corroded orifice bodies) that can make it aggravating to achieve peak performance. But it is still achieveable with some patience and a little finger pressure in the right area during assembly. The relatively soft plastic orifice (compared to one made of chrome plated brass) also created the potential for moronic techs to damage the orifice during installation or annual service which then made getting the poppet to seat perfectly nearly impossible. And the moronic tech of course was then far more likely to blame the reg than their own incompetence.
Some of the very late production D400's used a new lever design that was marketed as increasing flow rate, but in my experience it seems to do the opposite and results in slightly higher effort being required to sustain a high flow rate. I suspect this change actually addressed a CE freeflow resistance requirement.
Between the engineering hits the D400 took over time to meet CE freeflow requirements and due to the complaints of techs who were not familiar with it, SP discontinued the D400 for the 2004 season (after about 20 years of production). This is, in my opinion, one of the biggest mistakes SP ever made, especially when you consider that the X650 was not really ready for release yet, and is still essentially on indefinite backorder. The D400 was (and still is) arguably the best performing second stage Scubapro ever made.
