Here is what I have researched: Fosi seems to have QC issue. A lot of complaint about it due to bad tubes although the sound might be good (as you have described.) One user feedback in particular "they’re dealing with “vintage” tubes which aren’t being matched across the channels / across the tubes and the tubes are old / not in ideal shape so the failure rate is higher." It seems an OTL that requires all tubes to do the jobs that a xformer used to handle, which may not like demanding speaker like 4 ohm rated S400 II I owned. What is your experience?
for $200 IDK how they make some of these things, The GR70 I have is fantastic. it shoots way above its price way way above...I'm sure there probably is QC issues and maybe some of the tubes don't match, its $200, new tubes for this are cheap....its built better than a lot of gear twice its price....the build on the GR70 is really solid. ..
I'm gonna try upping the power supply because it seems like a obvious weak spot with the little wall wort...I picked out one on Amazon that should be a good upgrade and will order it today. I've only had it a few months.
but its $200 , for me the stock tubes it came ith sound great, but I'll look more into upgrades, I checked before but I'm happy with the sound.
its not an overly powerful unit....my Sennheiser HD660s and Koss proaa4 are about as hard to drive as you'll want to put on it...as a preamp maybe it only has 8v gain tops ,,,,,but its sounds rich, its lush. it sounds ZERO like all other fosi and aiyima products.
It amazed me hooked recently to my Laserdisc player playing Yusuko Agawa with Ron Carter quartet, absolutely sublime what it did with Ron Carters bass.