Hi! Welcome to HFGF!
I’ll venture the hypothesis that you’re pushing those amplifiers past their comfort zones. Since the issue is showing up on multiple sets of 1990, and I haven’t heard of this issue much, I don’t think it’s the 1990 (a quick google didn’t turn up that much on the issue, and where it did, it tended to be on cheaper amps). I’ve also reviewed the Zen CAN and have an Atom amp (V1). The Zen CAN - and I’ve taken heat for this but despite all the heat, it’s still true - does not handle hi-Z dynamics well in the bass. I got it to noticeably distort and struggle in the bass with Senns and Beyers. I don’t think its power supply is up to the task. The M50x and Sundara are easier loads and more where the CAN is comfortable.
I don’t remember having such problems with the Atom, but I didn’t use Hi-Z’s on it that much or drive them very hard when I did. However, as highly as I think of that amp (it’s great for what it is), it’s still a $99 amp and is going to be limited in what it can do.
Dynamic driver headphones tend to increase in impedance around 60 Hz. I don’t know what exactly the 1990 does, but I can guarantee that under 100 Hz its impedance is much higher than 250 Ohms. It’s going to be a difficult load for an entry amp in the bass range. My guess is you’re probably just learning that the hard way.
Hope that helps. 