I've Martin FX (flexible cores)lts. (12-54)on my 412 CE and my GS6E. They sound great on the 412 and just OK on the GS6.Maybe the GS needs that little extra tension.I'm gonna tweak the GS this weekend.Raise the action from 4/64 to 5/64, or maybe change the shims to get a little more saddle w/the lower action and put on some D'Addario EXP 80/20 lt.top/med. bottom(12-56).I love the way "Bluegrass" strings sound on dreadnaught guitars, so the GS6 should be similar.The low action on the GS6 is a dream to play,but the cannon sound on the bass notes needs a little more powder.
Steve