Dual monitors - Unable to change screen refresh rate (which is stopping one of the monitors working). Please help?

Summery:

I'm trying to use two samsung monitors with a new Mac Pro.

Monitor A is connected via DVI-D cable to Mini display port adaptor and into the Mini display port.

Monitor B is connected via the standard VGA cable, to an adaptor and into the DVI slot.

When both are plugged in, Monitor B switches to secondary display (displaying the display setup etc), Monitor A does not work (signal problem).

I've come to the conclusion that this is because the refresh rate has changed from 60 to 75 hertz and Monitor A cannot handle this.

The problem is I cannot CHANGE this setting, because the monitor isn't working! The menu to change it would be up as soon as I plugged in the monitors (the menu becomes specific for each display). I'm fairly sure if I could change it to 60 hertz it would start working.

Any way to change the refresh rate for each monitor without them being plugged in / displaying?

Many thanks.

More detailed information:

Monitor A is more advanced then monitor B.

Monitor B only has a standard VGA connector, so I have that going into an adaptor and into the Mac Pro's standard DVI.

I USED to connect monitor A that way (and it worked fine) but as monitor B only has that option I had to change.