DAC/Pre-Amp Upgrade, R2R or Delta Sigma?

think you should consider the Denafrips line of R2R DACs. Some of the best I have heard until you get to the 5 figure price range.

I would look at an FPGA DAC like the Chord Hugo TT2 or a DAVE.  My personal preference and would say they are better than anything I have heard other than Esoteric or dCS stacks.  I became a Chord dealer I like them so much.  

Global difference in sound signature, not sure what that means. As long as a DAC's noise and distortion is somewhere around - 120 there isn't going to be much of a noticable difference. The thing is R2R DACs  tend to not measure well so some will roll off the highs and produce noticable distortion which I imagine is where their " sound signature " would come in. There are of course lousy implemented Delta Sigma DACs too but a well implemented one like a Benchmark DAC3 doesn't really have a sound signature it's considered very neutral so what' goes in is what comes out. Good recordings sound good bad ones bad. Try to narrow down to a couple and get a friend to switch them back and forth and pick the one that sounds better to you using only your ears not your eyes.