I played the 99s with pros pro concept 1.33 at 55lb today. I ended up playing 4 sets (3 hours) with a 4.0 power hitter. Although the strings started getting stuck out of place a bit after 20 mins, the tension held up better than anything I have put in the racket thus far. The racket lost maybe 3lb in the first hour and one more lb over the last 2 hours. Will be trying concept 1.38 from bigt.