Finally I've found the right set up for X-Dual Gold 99, +10g butt cap, 3g weight slide at 12, double overgrip over 4-1/4 (normally I play 4-3/
and hybrid string at 55 lbs. The racquet plays very soft throughout the entire frame and this reminds me of Head PT57A. For sure, it's not 67RA as printed inside the throat. I think the Gold 99 may be just lighter and thinner version of PT57A (while Platinum 99 can match to PT57E). It's one of the most comfortable I've played. The way Gold 99 respond to hard-hitting ball is very well cushioned and comfortable, almost zero shock. Without 3g weight slide, the frame vibrates on off-center hit. No need to add more weight at 3/9 since the racquet is already torsionally stable.
The string I tried is also a dual core contruction. The outer core is poly and inner core is multifilament.