Well, there is one thing to list the wrong spec while another to list an incomplete spec. What iBasso did is the later. Technically they are not wrong but the information they provided is so limited that it doesn't serve any good to the buyer. Unfortunately it is all too common for most manufacturer to leave in blank and way more convenience to let the buyer fill them in with assumption that most likely won't be true. Yes, you can say it is a way of marketing.
Fiio has published the max. current output of their DAP, X5 is rated at >150 mA, X3 at > 250 mA, so X3 has significantly better driving power when compare to X5?
Coming from a traditional HiFi setup background, I certainly appreciate the amount of current that an amplifier can put out determine its driving power, I guess the same will hold the same in HeadFi DAP design. The obvious index in traditional HiFi game is whether an amplifier can double its output when the impedance drop by half (8ohm vs 4 ohm vs 2 ohm), I wonder is there an easy way out to speculate the current capacity of a DAP or headphone amplifier based on published specifications.
So DX50 high gain is hardly 3.1V as published? You didn't specify the gain setting of X5 and DX50 in your measurements, I assume both units are set in high gain from the figures. In fact, even Fiio website didn't specify the gain setting regarding their DAP output, I have asked here before with no luck, wonder if this is too much trouble for you to measure the X5 output in both LO and Hi gain setting, much appreciated in advance.
For raw driving power, I think X3 and X5 are both way above the rest and really pushing to the same level as a good dedicated portable amp. I won't say X3 is 'significantly' more power than X5, but I think it is safe to say both have way more than you need to get good result as long as you are not driving something like planner magnetic.
Both DAP are tested on high gain and max volume. I never get 3.1V on DX50 but I'll guess you will probably need a 75ohm load or higher. The problem with speculating power from spec is that some manufacturer uses uncommon load (or worst, no load at all) to get a good looking spec that doesn't hold up in the real world. For example, most DAP will measure beautifully without any load. Even a 150ohm load will look very well. But put a 16ohm load in, the result can be very poor, as lower impedance load is really going to stress the DAP and tell you what the hardware responses under everyday condition, given 10 ohm multi-driver CIEM isn't uncommon these days among audiophiles. In short, it is always better to take the measurement rather than hoping for the best.
Low gain max at 1.6Vrms, same 288mA current output into 47ohm load.