Hi Sam,
Reference line is having more MCPS regulators on the motherboard and the dual-DAC design, indeed. This means that inside the case, Reference-series has dual-mono power rails, but also dual-mono DAC implementation. Performance-series, like Conductor 3XP and Soloist 3XP, have their analogue circuitry powered via two MCPS regulators (paralleled across the internal amplifiers), while the Conductor 3/3X Reference have the analogue circuitry powered via four MCPS regulators (dual-mono).
The above doesn't means that one is a better performer than another, but it's more about the internal design, nothing else, hence the higher price on the Reference series.
I don't think I can get a Composer to compare it with C3X Ref., but I don't think anyone will be able to spot any differences between the two, in a well-conducted A/B test, because they share the same DAC chip, the same internal topology and same output-stage design.
From a measurement perspective, Reference and Composer series are having pretty much the same specs, in regard with THD and channel separation, so I guess we should choose the right device for us based on size, look & feel and maybe based on the internal design too. After all, our speakers are having the 2nd harmonic somewhere in between 50 and 60 dB away from the fundamental, so that would be a THD worse than 0.1%, while the subwoofer has usually a THD worse than 1% when passing the 90 dB SPL.

Also, some folks would prefer a single combo unit, some others will prefer dedicated DAC and separate headamp.