I also have a Nippon fiddle from the late teens. It is also very bright and powerful, and I've been through exactly what you are asking. Here are some things I've tried. I had a shop install a viola sound post(a lttle thicker), this helped only marginally, but did help. I did a different bridge on my own and left it thicker, this also helped a bit. I made sure the after length(of the strings behind the bridge, was in the 57 mm range), only marginal difference but still moving in the right direction. I've had Dominants on it. man are they loud and bright, and they won't take the retuning very well, unless you're just going to leave it cross tuned. I've had Pro Artes on it, they were flat out awful. I had Helicore mediums on it, the tone was better, then I tried Helicore Heavies, even better. The last time I changed strings on it, I tried Prim Orchestra, with a Lisa E(also from Prim). They're hard to find, look at Southwest Strings or Shar Music. The Prim Orchestra's sound the best of anything and will last well and take the retuning. I also still always have a roll up mute on the thing, and almost always it's just barely touching the backside of the bridge. It has plenty of volume this way and balanced good tone. It's still not necessarily my favorite instrument to play, but when I play it people tell me they like the sound out in front of it. Maybe it's just a thing where they sound the way they do, right under your ear for some reason.
I hope a few of these suggestions might help, but definitely synthetic core strings do not hold up nearly as well to retuning as steel strings do. Good Luck, let us know if you find a good combination for it. One of the great things about the fiddle is there is always more to learn.