Given everything you've said, and with the understanding that I've never heard the SR71B, I'd say you would be content with the less expensive 2Stepdance - especially since I think the Stepdance can drive all of your phones, just fine.
You might want to bring this to the attention of Ianmedium, however (with a PM). He's a big fan of the Stepdance, but has since upgraded to the SR71B (and went balanced). He has a lot of experience (hundreds of hours) with both amps, using the same source (CLAS + Solo) and the same headphones (LCD-2). He's the goto guy for anyone trying to decide between Stepdance and SR71B and I long ago came to trust his impressions.