I've been in the sub 20lb category before. Just not enough control for my type of swing. I think high 30's or low 40's is about as low as I'll go now.
-ok, so in the "
30's or low 40's" range, here are my toughts (based on your strings mentioned above)
-volkl cyclone 17, would provably be a good option, heard its on the stiffer side, so it should offer good control and feel comfortable/ish
-tier one ghost wire 19, IMO not a good option being soo thin, i owuld provably save this one for a 18/20 frame myself
-hyper g 17
and 19g , would provably feel as good as the cyclone, i would provably do a side/side test on with these 2 (cyclone/hyp-g) at the tension you choose,
-yonex pro poly tour 17, would provably feel the best at low tensions, (for me) in the initial stage of the hitting, (first week or so) but then as it loosens up it would be a bit too-soft/mushy (to keep a long time; 3 weeks,, etc,, not sure how long you keep your strings on a racquet)
-pros pro red devil, i find a bit too soft to venture into 30lbs with it, would become a trampoline too quickly to maintain good control,, IMO
-pros pro black force , is a good/GREAT option string to try a tension test with!! on any frame, good $ test (without wasting cash); while this is a slightly softer option than cyclone/hyp-g, it would narrow the tension options for me