As another owner of both the HE-1 and the Stealth, I fully agree with Nomax. If the tonality of the headphone is important to you, the Stealth is the way to go, imo. The HE-1 is still way better overall and with its huge transducer its sound is somehow mighty, bombastic compared to other headphone setups. It gives me peace of mind for the next 10+ years until my hearing will get worse anyway. But at least in some ways the Stealth comes quite close. Tonality is close but with the HE-1 having more treble sparkle. Bass definition of the Stealth is great when driven properly (with the Questyle CMA 15 in my case) and the Stealth does not have a real weakness unlike most other flagship headphones imho. Unfortunately, the stock cable holds it back a bit regarding bass definition and perceived speed. Zähl HM1 would give even more great options, but as an owner of the HE-1 I cannot justify pouring much money into a second headphone setup which will fall short compared to the HE-1 anyway.
I would not go for the X9000 without the possibility to use DSP/EQ (it has too little bass for my liking). And in case that you want to adjust its frequency response to the Harman Target, it ought to be tested before how much volume can be reached with the transducer.
Having seen Nomax' video with the one and only Axel Grell already, a significant design difference between the HE-1 and most other electrostatic (Stax) headphones becomes obvious. The video is in german language, but maybe folks here can learn some german in the course of the summer until he is willing to share it.