Is leaving a soprano in D tuning (hi A) safe with Martin 600s? Or do I need to get strings optimized for the higher tuning?.If new strings are best, any fluorocarbon suggestions?
You'll be fine tuning your flouros up - lots of fluoro brands have the same strings for soprano and concert so you know the extra tension will be fine.
I'm with Bill here, though. I use flouros on all of my ukes except my D tuned soprano - much prefer nylon strings for that. As nylon is usually lower tension you get to keep the mellow soprano string tension with the higher tuning.
Thanks for your input, folks! I just got a Koa Kiwaya soprano that sounds magical in D with the Martins I put on it. I'll seek out nylon strings for D tuning and try them out as well.
To reduce tension, you could try Worth CL (clear light) or BL (brown light) strings. You might also ask Ken Middleton (of Living Water) which gauges he recommends for D tuning; he may be able to make up custom sets.
When I get the courage, I'll put a set of gut strings on a 100 year old Martin then tun it to D just to hear some old ragtime. If the bridge pops off or a slot blows out, I can fix that. If a string breaks, its the end of that trial.