Had one for a few weeks and did a lot of testing. The Hifiman HE4XX from Drop sounds amazing, as does the Shure 1840, both get into the 60 percent volume range in high gain mode but sound so good when fully powered with this. I'm using an original Atom DAC, but also tried the Aune S6 and Khadas tone board using the RCA connectors. The Atom and Aune S6 were both great for music or gaming, and the Tone board was the best for music but not as good for gaming, but wow does it deliver rich bass when paired with the SP200.
No noise floor issues using RCA, might get a pair of RCA to XLR just for kicks to see. The Hifiman HE4xx sounds so good on this amp and also the Heresy. Make sure you do not use Monoprice cables, I have two pairs and they both sounded terrible, switching to the Cablesonline or the Kablesdirekt ones made it sound so much better.
If you have any low quality music files, you'll never stop hearing about it with this, it's very revealing. I have to test audio equipment that comes in all the time so I mainly got it for the XLR plug and equal power on both, not just a crippled 6.5mm plug. I have the Drop THX One amp here as well for testing, it's also really good. The main thing is if you need the low gain setting, then the Drop is a better choice, but if you need the feeling of almost unlimited powah then the SP200 is best. It's not truly balanced but it's really flexible for someone who needs the XLR plugs to test whatever comes in. Also I put a strip of black matte vinyl car wrap over the SP200 volume wheel and it's smooth and looks factory. Like the Heresy the grooves go the wrong way to provide grip, easy fix though.