FirstWatt J2

The single biggest issue is magnetic induction - those tiny little transformers are in the same metal case as a big-honkin' toroid, which even if shielded, has magnetic flux all around. So in a perfect world, the input transformers are both on the same side of the case, and as far from the power transformer as possible - this means, most likely, a mirror set of PCB.

The F6 production pieces put the little transformers to the rear and the big
transformer to the front, as with the M2 layout.

Best noise figures I get are <100 uV output noise (20-20K unweighted), and
worst case <200 uV. This is pretty good, but is typically achieved by rotating
the big transformer a bit until the noise is equal in both ch.

If you add shielding, You can do better. You can make a steel band for the
big transformer, and put some mu metal over the little transformers and get
about 6 dB reduction.

😎
 
I've got two J2s which I used for an active crossover setup, but now I only need two channels.

Is there any reason not to run them as monoblocks or brigded?
Could it be harmful to short the XLRs to get more output on a single RCA-channel? Like with the F4s.