Ultegra 6600 would be 10spd I think, 9 and 10 should be interchangeable for cranks/rings so you're fine if everything else is 9spd.

Looks like 6600 was the same 53/39/30 setup, you could look for that middle ring as direct replacement. If you throw Shimano 39t triple in ebay you actually get some good results: https://www.ebay.com/sch/i.html?_fro...LH_TitleDesc=0