Howdy Y'all,  (that's my best southern drawl - that I can type anyway

A crazy Minnesota Yankee (me) will be visiting soon and I have done some cursory research (BP / Eros / P411 etc etc)  it seems that most Asian providers are linked to/work out of AMP's - is this correct?

Most ads state 40/50/60 for services + tip.  So my question is this:
Is the 'tip' a set amount they tell you or is it what you want to give?
What is an average 'tip' for these places?
Anyone ever do a double at any of these places?
Any of these to stay away from or are any favorites of you guys?

Thanks in Advance & I am looking forward to sampling the wares.....