Running Legacy Software on 10.6.8 upgrading with X5690 3.46 GHz

Hi Guys,

I recently bought an old Mac Pro 2010 specifically for running snow leopard so I can use some legacy software (I'm a composer working on a documentary using 80s footage). I wanted to upgrade the CPU from 2.8 to 3.46 but I've hit a snag. The upgrade went fine - the Mac Pro boots into High Sierra just fine - but it no longer boots into Snow Leopard. (It booted into Snow Leopard when the CPU was a quad 2.8) - What's the story? Anyone know what the problem is? I think it might be that the X5690 was originally for the server models and therefore 10.6.8 won't work unless it is server 10.6.8?? that's one thought. Or maybe it's something simple and I'm missing a driver. Any help would be greatly appreciated. Thanks, Dylan