I'm really enjoying the M400EX so far - it sounds really great. Truthfully, it sounds a lot like the D70s (dual 4497 chips) and I doubt I'd be able to differentiate them in a blind test. I find that the D400EX seems to really exemplify the "relaxed" AKM sound and treble is perfect to my treble-sensitive ears. Definitely not saying that it lacks in any dynamics, but my completely subjective and probably biased opinion is that AKM DACs sound more refined and pleasurable to my ears, especially in the treble region. I am planning to A/B the M400EX against my Topping D50s which uses an ESS chip, so we'll see if that holds up in a side-by-side, albeit not blind, testing environment.
As it relates to the Topping D70s. I really liked that DAC too. However, I had all sorts of issues with the firmware and drivers. I'd get brief pauses in MQA playback all the time and my computer would forget the DAC after going to sleep (despite me adjusting every setting I could think of). I experience none of these issues with the SMSL so far. Regarding output voltage for RCA vs. Balanced, I'm honestly not sure. I thought I had seem somewhere that the voltages were more in line with industry standard (unlike the D70s), but I haven't been able to find that source so might have been a dream.