There may be some confusion here. The -15v control voltage you refer to is probably the "power supply" to the control desk. All tempus desks (as far as I'm aware) produced a "-10v" control voltage derived off the -15v supply rail. There may be a few that were modified for +ve operatin but they are very rare.
The answer to your question is that the Perma 12 will and do accept a -10v control voltage (as supplied by your 36 way desk) so I there should be no problem (that I can see) to use a demux with a -10v output voltage.
The only thing you may need to do is trim the dimmers after you change over.
The Perma 12 have 2 adjustments (from memory, top and bottom set) to compensate for varying input voltages. So what people have told you about it being OK is true, also the possibility of reduced lights is also true but can be overcome by adjusting the top and bottom sets.
If you need anymore clarification, give me a buzz at White Light on 0208 254 4740.