I'm 5'11 and have my stance set at 21" which is the narrowest or closest stance I can set on my 161cm board, But it still feels a bit too wide! Bindings are set at +18 / -9 degrees and I havent been able to try it out on the mountain yet, just how it feels set up at home.
Am I just a short legged freak or what!? I figured at 5'11 and 81kgs a 161cm board would be ideal. Sucks if I have to buy a shorter board...
With 81 kg, a 161 should be just fine for freeriding with a bit of freestyle in it.
I'm also 5'11, but lighter than you. One of my board is a 156 Prospect ltd 156 and I use it for anything other than really dip powder and/or hard carving. I also have a Rossignol Storm 162 which I use for the powder/groomers carving days.
On the prospect I often rock a 23,5 stance with +15 -15 angles. On the rossi I am at 23 with +18 -9.
My bet: you may not be shorter-legged than me but you probably tend to ride a bit too much stiff-legged and/or with bindings set with spoilers not parallel to edges and a bit too much forward lean.
Large duck stances usually require to ride low with not too much lean if any and spoilers as parallel to the edge as possible. At least that would explain why you feel more comfortable with the exact same stance width and a 0 angle (once your riding position and your binding tune is made so the spoilers are not in the way when board is flat, a duck should enable riding a wider stance than flat angles).