i think you have to ask yourself, what do you want more of? power? spin? control?
for me when i used it for a long time, i liked control so i strung it fairly tight (65 lbs) and i supplied the power and spin. of course i used a kevlar hybrid for a long time at that tight tension but i also felt that a good 15L topspin string worked for me since it took some pop from the racquet. the current string i have in my last psc6.1 (that i'm keeping around for sentimental reasons) is nxt tour 16 at 60 lbs. it's really not my cup of tea as it is too mushy.
When I got my Pro Staff 6.1, it came unstrung and I was itching to play with it. The only string I had around was a set of Prince Tournament Nylon. I strung it up at 57 lbs and surprisingly to me, I really liked the feel, control and power I received. I have been using natural gut in all of my other racquets (head classic, head i. radical, and head i prestige) and I love the feel of the Pro Staff 6.1 (85) with tournament nylon. I've gone through 2 string jobs so far with the set up and still love it.
I'm ordering some natural gut next week and I will try my 6.1 with NG for the first time. I am also interested in what other people suggest for stringing their Pro Staff 6.1s.
a freind of mine has used that frame for 10 years, now, strings it up with babolat vs gut at 65. if you can afford it, it doesnt get anybetter than that for power control, spin, and serving. tis a great stick, quite stiff though , so a soft playable strins helps. if you arent looking to spend that kind of money, go with a good myultifilament, like babolat premium excell, as it plays very well, and doesnt cost too much. i wouldnt go witha poly in this frame, its would be pretyt bad on the elbow over tiem, especially on mishits, and the sweetspot isnt large on that frame. softer multis or gut would be the way to go imo.
I have 6 6.1 classics. I like stringing them with Big bang ace 18, babolat gut 17, Wilson nxt 17, Goosen 18, Prince dura 18, Mix and match differnt strings. If fun to try differnt tensions and patterns so you dont get bored.
I tried a friend's PS 6.1 the other day. It felt really soft almost marshmallowy. But at the same time it had a lot of power. I asked him what he strung it with and what tension but he says it does not matter. He didn't want to disclose trade secrets I guess. I want to duplicate that in my racquet. Maybe I'll steal his racquet for one day and copy it. I really liked the feel and power.
48 klip gut in the mains. 50 forten sweet 16. With such a stiff lower powered frame i find that for me a softer string and lower tension that can create some pocketing and feel on the string works best.