I've been using the 99S for about a year now. I've tried a number of strings, some expensive, some not. I think I've finally found a winner in a string that I never would have suspected or tried, but did so at the behest of a friend of mine.
Prince SynGut/Poly Blend at $5.25 a pack is the best string I've tried. And I've tried:
- Wilson Spin Cycle
- Luxilon Ace
- Iso Speed Pulse
- Solinco Tour Bite Soft
- Volkl V-Rex
- Black Magic
- Luxilon 4G
- Tourna Big Hitter Black 7
- Tourna Big Hitter Blue
- Pacific ATP Power Poly Pro
What I like about the Prince SynGut/Poly Blend is:
- Price
- You can string it tight (within recommended range) and it doesn't kill you
- Ball cupping
- Control
- Power
- Touch
- Spin
- Durability

I actually keep a spreadsheet of my racquets with comments about the different strings, tension, date strung, etc. So if anyone has a question about what I didn't like, I have a comment at the ready.
Where I was getting 3 sets of play with BHB7, I've played 9 sets with this set up and there is just a bit of notching on the synthetic. The string combination gives you all the good things you expect from a non-poly setup like ball feel and cupping with none of the drawbacks, no feel, high impact, etc. I used to use Luxilon Ace with TF NRG 17. I thought the NRG would be a better selection than a plain jane synthetic, but I don't think so anymore. If you're wondering about a cross, take this for what it's worth, but I like synthetic gut in the crosses as much or more than anything else I've tried.
The Tournament Poly in 17 with the Synthetic gut gives, hands down, the best feel of anything I've tried in this frame to date. I have a few sets of the Tournament Poly 16 that I intend on trying as well again with a synthetic gut cross. And, I have to say that the cost of this string is a definite bonus. If I like the 16 gauge version, the costs are unbelievably low even compared to the $5.25 price tag for the prepackaged set. If you get a reel of the 16 poly and a reel of PSG 17, your stringing costs goes from $5.25 to $3.61. The difference between the two is enough to purchase another set of PSG.
The increase in tension has also benefited me serving and around the net. I'm still adjusting my ground strokes a bit but they are very good with this set up.