I thought I'd give this thread a little bit of an update.
For those who may have read my impressions between the Rx Mk3 vs SR-71B may remember that I wasn't very impressed with the SR-71B. That test was with a single ended earphone but later I tested with balanced earphones and still felt the SR-71B lacked transparency and clarity.
Recently a new balanced DAC was released by CypherLabs known as the Algorhythm Solo -dB. Despite some balance issues, with guidance from relevant involved parties I've managed to implement a temporary fix as a test; and one of the tests involves trying different balanced amps with the CLAS -dB. The interim-fixed CLAS -dB works well with the SR-71B, and this is the first time I've actually been thoroughly pleased with my SR-71B. The clarity improved, the bass extension is clearly more apparent and so is the detail.
Where I previously mentioned the SR-71B mids and vocals felt rather muffled, that is all gone. Now with the backend balanced, the difference between the SR-71B and Rx Mk3 feels more to do with sonic preferences rather than superiority of competing products.
At least with the 3xIEMs I can switch back 'n forth between single ended and balancing, the balancing on these IEM/headphone side yielded minor improvements. Whereas (now I can test with the CLAS -dB) the backend balancing of the ICs appear to be a bigger improvement.
My thoughts are that the only way for me to enjoy the SR-71B is to have a complete balanced setup right through.