Replacing your present source (mac mini + DA conversion in BP25) with BDP-1 + BDA-1 will without any doubt result in significant performance enhancements, there will be less jitter induced problems, not to mention that BDA-1 is exceptionally well executed converter in all areas that matter, what results in very audible improvements.
On the other hand IB2 powered with 14BSST instead of 4BSST will also show significant benefits in many areas, not only in bass control that is described most commonly as benefit of more power on IB2, but more important to me across the entire range, with better dynamics, larger soundstage and imaging.
I would however go for 2X 7BSST (or SST2) rather than 14B of any version, for a bit more $ you get much better performance.
I had or still have most of the components mentioned, so it is all firsthand experience.
Hard to say what to do first, can you not do both?
