The N12 does not have high end preamps. Don't believe everything you read, especially advertising copy from manufacturers trying to sell a product.
As far as the X.Sum, I own one of those along with a couple Xtramix's. The X.Sum is a great sounding little one rack space summing mixer. However, it doesn't have any aux sends or busses (it does have an alternate buss making it a sort of four buss mixer). It also doesn't have any eq built in.
If you are serious about doing a real analog mix, the X.Sum is not the way to go. I use it just the way Fishmed describes, i.e. I use plugins and then outboard between the DAW and the X.Sum. But that is very different than doing a full-on analog mix. For that you really do need a board with 8 busses and eq, aux sends/returns, etc. And that sounds like what you are looking for.
I also owned three Mackie LM-3204's at one point. It's actually one of their better sounding mixers due to the extreme simplicity of the signal flow, but again it's very limited as to how detailed a mix you can do with it. I wouldn't recommend it for that, it's a keyboard submixer. If you go with Mackie, I wouldn't get less than the latest 16 channel Onyx.
But the Toft might the board for you. If you do decide to go that way, get the mixer first and use the onboard preamps for a while. You might find they suit you, and that will save you buying a pair of standalone preamps.