That's a tough one, mainly because I never owned an SR-71B myself, and the listening rigs were different (LCD-2 with both, but Mk.III had CLAS, SR-71B had CD transport). That said, the differences aren't night and day, and this one will probably come down to personal preference. To my ears, I felt that bass was comparable, and both rendered mids and detail admirably, but the Rx Mk.III did better in the high end. I really enjoyed female vocals through the Mk.III. I also thought the Mk.III did a bit better in terms of soundstage depth. Again, I wish I had the chance to hook the HE-6 up to both the SR-71B and the ALO Rx Mk.III, but alas, I didn't have the presence of mind to do that.