The 60 dollar price tag was discerning, but it seems that they are praised by many enthusiasts because of the price/performance ratio. How much better would the AD990xs be in comparison? (Factoring in build quality, comfort, and sound quality- which I understand is an amorphous subject)
This is what I think: coming from base level headphones, the difference between Pioneer Se-A1000 and ATH AD900x won't be that much really. Both should be more spacious, precise, realistic, and overall more enjoyable in comparison with your current headphones.
Going from Se-A1000 to AD900x, you should get a slightly better bass response, a bit more refined/clearer mids, and probably (not sure) smoother highs.
AD900x won't sound 3 times better than Se A1000, neither 2 times better, but slightly better. While they cost approx 3 times the price of Se A1000s, if you can afford them, I think you won't regret buying them, and you won't have the need (or at least the temptation) of upgrading for a long while. The extra refinement puts AD900x in the same league as some world class performers like DT880, HD600 and K70x.
If you don't want to spend the extra money, you won't be out of the party with your Pioneers.
Se-A1000s are confortable, natural sounding and provide a really wide soundstage for headphones.
Their build quality (cups, cable, plug, soft carrying bag, etc) is on the same level as those much more expensive headphones.
Comparing comfort is not wise because comfort is a pretty personal thing, but the general consensus is that both are pretty comfortable headphones. (Some people hate those ATH wings, some hate the auto ajustable headband, you know...)
Being not as resolving as some higher end headphones, Se A1000s manages to be more forgiving of poorly recorder/mastered material. I remember myself enjoying some tracks (with my Se A1000s) which I simply don't like with my K702.
The main weaknesses of Pionners are:
Bass is pretty natural sounding, but a bit loose.
Mids are somewhat veiled wich makes the wide soundstage a bit diffuse
Highs are a bit colored then they are great for some recordings and not that great for others.
At 60, you can't go wrong with Se-A1000s
At 200, you can't go wrong with AD900x
Oh, you can't go wrong... Lucky you!