Why.is the standard output 7 / 15 vrms? Thought the standard was 2 / 4
There is no such standard at 2/4Vrms as far as I know (at least not defined/governed by any standard organization). A lot of DACs especially ones targeting professional or studio use will have higher output levels (especially on XLR our) because pro equipment traditionally expects higher output levels. A lot of DACs do follow the 2/4Vrms levels as consumer equipment (such as self-powered speakers) cannot handle hotter signals without distorting/clipping. But for stand alone desktop sized amps, almost all of them should be able to handle the hotter outputs, at least up to 7-8Vrms for XLR (15 is a bit high).
DACs with relatively high output levels (6Vs+) on XLR include the Mytek, Calyx 24/192, Matrix X-Sabre, most MSB DACs, to name a few. Most Audio-GD DACs also output at 5Vrms. Haven't heard any issues with any of these amps (except the Mytek which before setting the jumpers to lower the RCA level it did cause clipping with my powered speakers).