I've been wondering about this as well, but if MiniDSP documents which chip's used on the board I'm not spotting where. Neither am I finding any threads which mention it or pics of the board without the heatsink on. But from the specs, supply voltage, and shape of the heatsink my guess is it might be a TAS5601.
Don't forget slew rate, GBP, PSRR, CMRR, and that while harmonic masking applies to THD it's not particularly relevant to IMD. Assuming a source with sufficient drive current, the 49713 beats everything I know of except the 49990 (though SEOs based on either op amp will generally performance limit on the output devices). However, that's a somewhat unfair comparison as it's tough to build one SEO channel for the price of the MiniAmp, much less four and the glue logic to attach to a MiniDSP---after all, a BOM of USD 5 per channel is insufficient to buy a single 49713 in DIY quantities.