Preferred strings for an Outdoor Soprano?

I've had an Outdoor Soprano for a couple of years, and haven't played it all that much. It's still got the original strings, and I'm wondering if a string upgrade might give it a little more volume, and maybe improve the tuning stability, as mine never seems to stay in tune.

Has anyone experimented and found a preferred string for these plastic workhorses?

Regular old Martin M600 strings sounded great on mine.

I have Fremont Blackline Mediums on my Outdoor Soprano, seemed to give it a bit more volume and a higher tension which I prefer.

Cheers,

Steve.