@aberyclark #1 should be easy enough to test. Find (or make) a quiet track and just plug your source straight into the J2. Then compare with it through the BHK. Or borrow a DAC with variable output and try the same.
As for #2, I haven't heard the J2, but I suspect it would sound quite different from the 25. The J2 and XA25 use different output devices, and a different amplification typology. The J2 has a SET-like distortion profile with 2nd harmonic dominance and the XA25 has a 3rd harmonic dominant profile (I believe this is true of the 60.8 as well). They likely have different damping factors too, so may interact with your speakers differently. Only one way to know which you'd prefer.