I have used many different password protection systems...
ProxyPass has a web/secret word auth system now to help add protection, it works with pretty much every plugin system. I haven't ever tested Phantom Frog.
StrongBox has worked with all but one feed provider that I have tried. The older 'referral' based plugins, besides being impossible to protect, simply need a wild card referrer added in, like %.members.paysite.com or *.members.paysite.com - If you have any problems with getting a feed to work you should email ray if the content provider isn't able to help.