Sorry--meant to reply to you earlier! I did work directly with Castagnari. I ordered it back in late April and received it mid-October (they take a month off in Italy, which is very civilized). Build quality is excellent - the Serafini is also well built, but seems "slight" in comparison--less dense, but that also makes it more nimble. The reed quality on both are great, but the Castagnari might have slightly more usable range in the upper end, where it is pleasing (i.e. not shrill) all the way up on the M reeds and LMM.
As for the range, If you've seen some of my posts or my channels you'll know that I play folk (Irish Trad, Flemish, English, Breton) as well as an insanely diverse set of repertoire not commonly associated with the accordion--Radiohead, Nine Inch Nails, Bjork, Bowie, Kraftwerk, Steely Dan, Velvet Underground, Smashing Pumpkins, Aphex Twin, Nick Drake, Stereolab., Beatles, Amy Winehouse to mention a few. I haven't felt constrained by either box, so 11-12-11 seems to work for me. Certain tunes do need a little forethought as to where I want to start them (or transpose).
When asked which box I prefer, my honest answer is "the one I'm currently playing"as each one is a world of endless inspiration in less than cubic foot. The Serafini is more minimalist and quick/agile which lends itself quick changes in direction, rhythmic flourishes, while the Handry excels at longer drones, phrases. and has a slightly deeper, richer sound. I can't speak for anyone else, but they both seem to hit the perfect sweet spot of size, range and reed combinations that work for me...zero box envy/lust at all.