Well starting a tube in this scenario in not really advisable unless you know how to market it effectively and u a pro in adult with enough knowledge on adult sub niches, well a micro niche tube is quite ok. Mainstream tubes requires much investment (even if you have much to invest, you will be disappointed when you don't see much revenue from it to cover up your cost) You have to invest 1000s of $ on link building yearly (i sell two links on my pr3 and pr1 pages for 100$/year linking to one among the top) so literally its a million dollar investment to make up something pro and few k if you dont want it to be pro looking, but risky business. My friend is having one small with 15k - 20k traffic per day and his expenses are pretty high..