The highest I tried to listen with the X1 is 85, at this level (70dB avg, 83dB peak) I don't hear any distortion but its very uncomfortable and I can only listen for a few seconds.
My HE-400i is an older model with the SMC connectors and the back plate, it has been modded with sorbothane in the cups and on the back plate, also have done the grill mod so it does sound different from stock. There is also some slight differences between the old model with the back plate and the newer one without the back plate, can't remember what they were but hopefully someone else here can remember what it was.
FWIW I don't use my HE-400i or X1 much these days, been on a weird run of building custom Grado clones using some really nice new drivers from Elleven Acoustica and Nhoord. I know you dislike Grado's but these sound nothing like them, much more balanced and no hyped up highs to irritate anyone, here is the last one I've built: next to an EL8 for size comparison at 1/3 the weight, only thing the EL8 does better is that it has slightly more the sub-bass.
A picture of the driver with a modified cup and SMC jacks for removable cables.