I bought a HD15 VGA / HD15 VGA buffer from axel81 recently that is precisely designed to plug into the RGB adapter you have. It relies on 5v power being supplied on pin 9 of the Unbuffered RGB adapter. He modified his RGB adapters to draw 5v from pin 23 of the Amiga RGB port to pin 9 of the VGA out in his latest revision 2, so if you bought yours recently this would be it. Pin 9 is not usually used from online sources. The Amiga H-sync pin and V-sync pin also supply 3v and .9v respectively.
There is no guarantee that a particular product is going to have the same pinout. There are a few port pins required to be so but the others it's open. If you have power going somewhere it shouldn't the Amiga isn't going to work. Another thing is if the VGA switches are allowing the monitor to draw power where it shouldn't; your PSU may not be able to supply enough amps to get the system and HDD up and running.
