when I have designed APP2050 the first thing I have tought was to make it compatible for +-24V supply because I think, as is natural to do, that -+24V of power is necessary to have >24dbu out with a 1:1 trasformer or a simple trasformerless out.
This is the reason of Jensen regarding the 990 for example (I suppose). But the market is strange because many customers don't look inside their machines so I think is important that someone makes a "real" -+24V gear as, for example, a transparent sound preamp. If many gears run at +-16V , even if without a real trasparent sound look, because to have a +-24V standard? (several customers think)
I think that GroupDIY51X standard is better than 500 but I am an EE.