Gut/poly hybrids feel amazing in this frame. The only issue I have with it is durability. I'm only getting about 6 hours on them before the gut snaps. This is with 16g Prince gut mains.
I am testing full beds of Silverstring and Iontec salmon both in 1.25 and am really enjoying the Iontec. It's got a real nice solid feel without being to trampoliney(I know it's not a real word but that's the only way I could put it) like other polys I've tried. Not as spinny as gut/poly but way more than enough for me.
Overall, I am really liking these sticks. The spin is nice and the sweetspot is huge but the biggest reason that I will be sticking with these racquets is the comfort. The 99S is one of only 2 racquets that I can play with a full bed of poly. The EXO3 Tour is the other.