Personally, I think reg design in general and second stage design in particular peaked in the late 1980s to early 1990s, and that peak occurred with Scubapro designs.
The Mk 10 Balanced Adjustable and slightly later Mk 10 G250 was hard to beat, and with a Mk 10 Plus upgrade the first stage still delivers more than enough gas for any technical diving application and the G250 (along with the G250V) is still a technical diving gold standard - one basically copied by Aqualung in the Legend and in the Apeks second stages as well as Hog and a few other companies in various degrees of quality.
The D400, in it's original pre-1994 metal one piece aspirator and orifice, pre-CE certified configuration was, in my opinion the most awesome production reg ever made, one that offered superb subjective breathing performance - performance not really reflected on WOB charts, given how those are manipulated. Sadly, Scubapro essentially detuned it to meet CE free flow requirements and saddled it with a plastic orifice for reasons unknown, then killed it as it was different and poorly techs had trouble tuning it properly.
The slightly later Mk 20 first stage was great for higher pressure applications, once they worked out the piston and seat bugs and the current Mk 25 offers no significant improvement on the final Mk 20 configuration.
Since then, the only new design that has really impressed me has been the Mk 17, but Scubapro still promotes the Mk 25 as their "flagship" model, in large part in deference to older than dirt dive shop owners who were steeped in the "balanced diaphragm regs are crap" propaganda and still hold that bias.
What has changed since the peak however, is that some other companies have improved and closed the gap that formerly existed. That reality, along with the economic need and the desire to sell new regs to old customers seems to drive the steady march of new models.
Personally, I think that would still be a valid approach, but Scubapro has went through a bad patch of the marketing folks driving the designs rather than divers and engineers and some of the products have been released a bit too soon, and with design goals that don't always adhere to the original meaning of the phrase "deep down you want the best". Smaller isn't always better, there is no substitute for quality and design decisions like the X650 still mystify me as they used the angled diaphragm of the D400, without the co-axial exhaust valve and with no apparently no understanding of how the two interacted to basically eliminate case geometry fault as a problem, given that the location of the exhaust valve in the X650 actually aggravated the problem.
It sounds like a negative but it really isn't as Scubapro produced some truly superb designs a decade or so ahead of the competition and at some point you run into the "where do we go from here?" problem. As noted above, smaller size was one approach but it's not all its cracked up to be and it's hard to improve on "old" designs that were already superb - especially when the company continues to retrofit new developments into the older designs. The Balanced Adjustable and original G250 for example, both benefit from the latest S-wing poppet used in the S600, G250V, etc, so rather than the designs aging into obsolescence, they continued to improve. That is indeed a unique challenge in terms of selling "new" regs.
Consequently, Scubapro is still my favorite company and I would not trade a Mk 17 G250V for anything else currently available from any reg company.
