La Bella Deep Talkin' Rounds might be worth a shot. I can't think of any other rounds that are voiced as mellow as Sun Beams, except for maybe GHS Boomers once broken in, but I have never liked Boomers myself.
I bought a set of Deep Talkin' Rounds a while back, but I never let them break in because I got the wrong gauge (too heavy). I remember liking the way sounded a lot though, big lows with definition, not very zingy for SS rounds. I will try them again one day, but the bass I was using them on is currently wearing DR flats which I won't be changing anytime soon. Good luck with the search.
Stuff I use:
Fender Am. Std. Jazz V
Fender MIM P-bass
Epifani PS112 (x2)
Spector Club #2; Bongo Club #12; Genz-Benz Club #20; Epifani Club #92; Carvin Club #218