According to the following calculator there is around 16.5% tax (on $100K income) ... you can compare it with bunch of company and it looks like Singapore is the lowest.
http://www.guidemehongkong.com/taxation/topics/hong-kong-tax...
http://www.guidemehongkong.com/taxation/topics/hong-kong-tax...
To pay no tax all your income needs to come from outside of HK in addition to not having a local operation.
http://www.guidemehongkong.com/taxation/corporate-tax/calcul...
btw, do you have any info on any agent who can help with this? do you need a physical address (even for opening a bank account)? Do you need to visit HK for any reason to setup a company?
The registered address in HK can be provided by the agent. I'm not sure what else you mean by physical address.
You'll need to visit HK to open a bank account in case you need one in HK, but you could open a bank account in any other country as well if you can find a bank willing to do so (tough where I'm from).
I have no idea about the legal intricacies involved for US citizens, but that doesn't change the fact that there is no corporate tax for the HK setup I described.