Will Spectral 15SS preamp work in my system?

Yes and you don't need MIT wires . Did it for years.
Joe

I am also considering Audionet Pre G2, any views?

It should work fine. And MIT interconnects aren't required, but MIT cables do match up well with Spectral gear. Both have similar strengths. I think you'd appreciate the strengths of the 15SS even more with MIT, though you may get lucky and find another brand that does just as well. Though why go through the crapshoot when you already know that the 15SS was designed with MIT in mind. I'm speaking from experience as I tried to avoid MIT with my 30S, but as my surrounding components improved, the Spectral/MIT synergy became more evident.

Also note that Spectral preamps often have very low input impedances. Many tube components have trouble with this. So you may want to think of another preamp if your long term plans include any source gear with tubes. This is the only negative that I encountered with my 30S, as two highly regarded phono stages couldn't drive it.