I use APO, but it's hardly a high quality EQ. Its advantages are that it's free and it works system-wide, so it will work on everything, whether games, youtube, etc. However, you will certainly hear a loss in transparency when you use high-Q filters, and especially when you stack many of them close together. However, if you use broader filters with lower Q (<3.5) you can work around it.
Basically, do you gain more in transparency from having a flat FR than you lose in using EQ? In many cases, the answer is that you do, as long as you use the least amount of EQ possible to get close to neutral.
EQing is a skill, and one that takes a while to learn. It's very easy to try to EQ, end up with something that sounds genuinely awful, and then say "yeah EQ sucks, no EQ is better" but the issue is that you're not doing it right, and learning how to takes a fairly long time. And not every headphone will respond well to it either, you want something with a smooth FR, few resonances, and low distortion, and you want an amp with enough headroom to run things above your normal listening volume and not clip, distort, or overheat, since when you boost frequencies you will also need to lower the total output so that the highest peak won't go above 0, or you risk clipping your source.
Of course, if you have a headphone where you like the FR already, you don't need to EQ. And if you are the sort of listener that likes the differences in headphones and you value them for the different presentations they provide, then you also probably don't need to EQ much if at all. For me though, I am looking for a certain sort of sound, and most headphones don't give it to me, so I could chase the dragon and spend thousands upon thousands looking for that perfect headphone - which I've done in the past - or I could just slap an EQ onto something I have already and get it reasonably close, and call it a day.
For Stax, at least for the modern Lambdas, they all have a peak in the mids somewhere around 1.2khz that makes the mids sound nasal, and knocking it out with EQ is pretty easy. The treble is mostly shrill due to emphasis at around 5.5khz or so, and knocking that out also makes the treble smoother. The upper mids in them are a bit recessed but how much really depends on you, some people like a more laid-back upper mid and some want it more present. The rest of the treble past 5-6khz you will need to do by ear since your own ear anatomy will affect what you hear, and you shouldn't try to EQ things to make them sound perfectly flat, just lower the egregious peaks and balance the overall levels. You don't need to get it perfect, you just need to get it close, and the less EQ you use, the less transparency you lose.
Regarding measurements, you can use measurements up to about 2khz, past then they become less reliable, and past 4-5khz it's a complete crapshoot and everybody hears differently. So if you start with a profile that's based on a measurement - it will probably be at least somewhat reliable up to 2khz, and past then, you'll need to make your own.
Lastly, bass extension in 'stats is mostly a seal issue, and fixing bass extension with EQ when you have a 'stat that won't seal properly is a fool's errand. It's much better to try and fix the seal, though not always possible without heavy mods.
