It's one of Erno's designs all right ;-) and that is not a bad thing.
Do you have a parts list to go with the schematic shown? I suspect the input FETs are the old favorite and still available (gott sei dank) Toshiba 2SK170/2SJ74 parts, which are wonderfully low noise, low pinchoff voltage. But what the cascoded FETs are I don't know---they have to have a pinchoff voltage magnitude larger than that of the input devices to produce enough drain-source voltage for them to operate, and have as well saturation drain current equal or better. I could rummage through some issues of Audio Amateur/Audio Electronics/audioXpress to find some Borbely articles and see what these parts probably are.
The attempt to do a fully complementary amp throughout is well-intentioned and largely successful, although Nature is cruel to us here, owing to the considerably lower mobility of holes compared to electrons in silicon. This means that, everything else equal, a P-channel FET like Q3,4 in the schematic will always have to be larger area and higher capacitance to have tranconductance and noise performance comparable to the N-channel devices. So there is an inherent asymmetry in the capacitances. Is this going to be a problem? Probably not---the input C's are doubly bootstrapped, the gate-source C's by the series feedback, and the drain-gate C's by the upper cascode FETs. And the voltage swing at the upper Q drains is to some extent reduced by the overall feedback as well.
Although the noise performance is exemplary (note how it degrades with even only a 50 ohm source Z---and that's not due to bias current shot noise folks, just 50 ohm thermal!) I believe there is a slightly more complicated topology that can reduce it even a bit more. Is it worth it? Well, probably not---but did the Audi A4 owners out there feel a burning desire to upgrade when the 3.0 liter engine models made them feel hopelessly emasculated in their mere 2.8 liter cars?
Erno clearly deserves his tariff on these designs, and helps to keep the community aware of the really great FETs. The semiconductor majors I am told gave up on the highest quality in these parts when they adopted ion implantation as the process of choice for impurity doping, in lieu of diffusion. It had little impact on most users and probably shaved a few hours off of the process, but the semi lattice and the resulting parts were never the same, despite the annealing cycle to try to put humpty dumpty back together again. The Toshiba parts continue to be fabricated with great care using diffusion I believe, but their worldwide comsumption is dwindling and parts are being discontinued left and right. To the possible rescue comes Linear Integrated Systems, who have just started to make a 2SK170 substitute and have plans for a dual part to replace the discontiuned Toshiba 2SK389.
Save the FET's!
Brad