-10 and +4 sure are odd choices for a "Pad" and a "Boost" as they do make it very confusing with the "+4dBu / -10dBv" standards. Even stranger coming from a reputable manufacturer like Dynaudio. +10 and -4 would seem to make more sense when looking at it from that angle :-)
The only drawback of running TotalMix at -20 is if a full-scale signal DOES happen to make it past Totalmix - you can probably kiss your tweeters goodbye (and maybe your woofers, too). You also lose a bit of SNR, but shouldn't really be losing any "resolution" per se.
FWIW - I feed a 4000Watt PA system and 1400Watts of monitor wedges directly from TotalMix (no passive attenuators or other volume controls between the DAC and the processor inputs) - and I've had very few issues over many years (nothing serious) - so as long as you know TotalMix well and are a little cautious - you should be fine...
MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8