The main issues were that the D400 was different to service and adjust, and over time lost much of its perfromance in order to meet a truly stupid CE freeflow resistance standard.
All of the other SP regs are either:
1. A single adjustment design where the position of the orifice (screwed further in or out) both controls the inhlation effort of the reg and controls the lever height. This applies to all the current balanced second stages Scubapro sells. Or,
2. A dual adjustment design where the orifice controls the inhlation effort and a nut on top of the poppet controls the lever height. This applies to all the unbalanced second tages Scubapro sells.
The D400 in contrast (and the Air 1, D300 and D350 that preceded it) uses a center balanced valve arrangement with a fixed orifice. Adjustment of the reg is easy but needs to be done in the proper order with the lever adjusted last and with just the right amount of play to keep the reg stable without unneccesarily increasing inhalation effort. When properly a done a pre-1998 D400 could deliver inhaltion efforts in the .6 to .7" of water range - extremely good by any standard.
In 1998 things started to go wrong for the D400 in the form of eurpoean CE freeflow resistance standards. This standard was quite frankly viewed by many as being very benefical to lower performing eurpoean models as passing it esentially required detuning the second stage, and in particular the D400 to the point where there was no discernable difference between an excellent breathing second stage like the D400 and an average second stage.
In 1999 this detuning process continued with a new lever that, as far as I can tell, just increased the inhalation effort needed to sustain high flow rates to around the 1.5" of water level, and in 2000 got worse with a move to a replaceable plastic orifice that was not as sharp as the original brass orifice.
The end result is that between tuning and lever changes to meet a totally pointless freeflow requirement and the move to the plastic orifice, the D400 lost a great deal of potential performance. In that regard the tech you spoke with was partially correct as its WOB numbers would have declined in order to meet the freeflow resistance requirements.
As a result by 2003-2004 when it was discontinued, it was not offerring the same level of performance it did when it introduced. The reason I heard at the time that it was discontinued was that Scubapro felt that the reg was a liabilty in terms of tech training requirements as it was so different than other second stages. The X650 that replaced it was designed a single adjustment design with a poppet nearly identical to other SP balanced regs to address this issue.
Another tech related issue issue is that the spring pressure in the D400 are much lighter and the seat develops a seating groove over a much longer period of time. The result of this is that with a D400 tuned fro minimum inhalation effort, it is not uncommon to have to tweak it a bit mid season to eliminate a mild freeflow that amy result once the seat develops a groove. Some custmers got cranky about this and that made dealers unhappy. The answer was again to detune it.
Another factor I think was that Scubapro never really pushed it as its premier reg and instead pushed the G250 and its derivatives. THe G250 is an excellent reg and is capable of great perormance and air flow, but the D400 had a quality to the airflow that was hard to match. Compariing the two is much like comparing a Camaro to a Jaguar, both can go fast, but the Jag has a lot more comfort and class.
