What do you mean by Desktop O2 board? I believe you can use the current board with a higher case enclosure to add RCA input and 1/4" headphone output.
Here is the info for the BOX ENCLOSURES B3-080 DESKTOP AMP: The B3-080
Weren't you the one complaining about shipping to the antipodes? You're way too far away to get one.
I was planning to sign up for a few boards (just in case, because my soldering "skills" are best suited to splicing wire...) but kept putting it off because I wasn't feeling very well. Fortunately I got a lucky message and I don't need to anymore. Hopefully I'll be feeling better by the time a group buy for the desktop O2 boards rolls around. After I get a desktop O2 I probably won't end up using this one very much and I'll likely give it away to someone in need of an amp.
I don't see a 2040 on E-Mu's website, did you mean the 0204? It says the line outs are +6.7dBV which should be 2.16VRMS so assuming you aren't going to use your inefficient 'phones with your DAP and only with the E-Mu then 1x and 3.1x (which is one of the "standard" gains) or 3.4x for maximum power if he'll do custom gains (from the 7.5/Input VRMS = max gain formula for the O2) should probably be good low and medium efficiency headphones from a DAP and both high and low efficiency 'phones from the E-Mu.
OTOH, some of those Etys are odd ducks for BA IEMs with very low efficiency. I've never owned either, but the specs say that the HD25s are more sensitive than the ER4S. That would give you more leeway than normal for BA IEMs but it still might be a problem if you like to listen softly.
Without an expensive stepped volume control of some kind there's not really any good way for an amp with only 2 gains to work perfectly with so many different combinations of sources and 'phones. With gains of 2.5x and 6.5x on mine I need to use the digital volume control on both my PC an D2+ (which doesn't have a line out anyway) with my SE530s but it also allows me to use my YH-3s with my D2+ at any sane volume I'd like. If you use a line out from a DAP you'll likely want a 1x gain since there will be no other way to lower the volume.
Its hard to say what to go with because no combination is perfect...