I have 2x Wilson Six.One Tour racquets. Both are strung @75lbs. I've played Pro Staff 6.0s & nCodes for >15 years, always @75lbs. without issues. I have noticed 3-5mm cracks on both Six.One racquets on the flat beam @approx. 7o'clock position. There is a silver cosmetic line, & the cracks are ~1/2 way between this stripe and the end corner of the yoke. Yes, I'm aware of the recommend 50-60lbs. recommend tension. Again, overstringing in excess of the recommended rating has never been an issue w/other Wilson racquets w/respect to comprising the structural integrity of the frame. Is the recommended tension a reference with respect to player feel (power v. control) or a "warning" with respect to compromise of the structural integrity. Thank you for any and all advice and responses in this regard.
Wilson Six.One Tour frame w/cracks?
i think nowadays the recommended tension range more refers to the material strength. i would also suggest you string your racquet at lower tension level because such high tension would void the first year warranty (if any) when buying new racquet. however, since you are very accustomed to your tension, it is up to you to keep stringing at 75 lbs 
i found myself preferring toward beyond the recommended lower tension limit. for example, on my Kobra Team FX i have the mains strung at 48lbs, to my liking
or you may want to try (at your own risks, of course) the liquidmetal racquets from Head? they are from about 5 years ago, but i think the material is stronger since they induced liquidmetal alloy to the graphite/carbon frame.
hope this helps
i found myself preferring toward beyond the recommended lower tension limit. for example, on my Kobra Team FX i have the mains strung at 48lbs, to my liking
or you may want to try (at your own risks, of course) the liquidmetal racquets from Head? they are from about 5 years ago, but i think the material is stronger since they induced liquidmetal alloy to the graphite/carbon frame.
hope this helps