12 string playability.

[Clint]

I seem to have a dull sounding main A string, though. Any thoughts on the cause?

It could just be a bum string. Maybe it's getting pinched. Check the nut slot and the bridge saddle.

[buchrob]

Temporarily install a thinner string in place of the offending A string. If it rings true, it's probably just a little sanding that's needed at the nut.

Factory nuts are pretty remarkably close, but may require a little tweaking, especially if using other strings.