Look who's got the fever for the flavor of the 6.1 Classics!!!
Can't say I've had success with any multi's in my 6.1's amigo, but you'd probably want one with more pop/crispness than marshmallow soft performance. I usually run 17 ga. syn. gut at 64-65 lbs. in those frames for an agreeable combo of crispness, control, power, etc. and I'd probably go just as tight if not a pound or two more with a multi.
I agree with the Tecnifibre Biphase, but you might do well to try a hybrid of that in the mains with a syn. gut in the crosses - that layout works really well for a teaching pro I know/play with on occasion. Tim at LaserFibre is a pal of mine and I've used some of his multi's, too. Laser Supreme has rather good service life and I absolutely love his Phenom XRC in my Prince NXG mids - for a multi, that string doesn't play especially rubbery (neither does the Biphase, imo) and I get nice overall performance with it. For the 6.1's, I'd go with the 16 ga. over the 17 ga. if you want some durability in the Wilson. By comparison, his Pro Stock is extremely soft stuff which I'd only use to beat tennis elbow or as a hybrid to make a really plush stringbed.
One other multi that impressed me was a set of Yonex 850 - don't remember if it was the regular or the "Pro" type - which came on a 2nd hand Yonex 002 Tour that I had for a while. Solid performer that seemed nice and durable - it didn't seem to break down at all over the few sessions that I used it. It was just a few pounds loose for me in that racquet, but it worked well. When I find an excuse in the future, I want to try this string again.
