I don't think there are many Aristaeus or Blue Hawaii owners who have heard both amps, so maybe the OP won't mind us including the ES-1 in the discussion.
Electrostatics are my favorite headphones, and the HE90 my overall favorite, so the Aristaeus vs. ES-1 is an interesting topic for me. I've owned the Aristaeus for a much longer time than the ES-1, one year vs. one month, so I'm more familiar with the Aristaeus and have had more experience tube-rolling it.
I think the two ECC83 positions affect the sound more than the four ECL86 tubes. I've tried quads of Telefunken, Mullard, Siemens, Valvo and Philips ECL86 with similar results, while the sound varies more when changing the ECC83 tubes. My favorite tubes for the HE90 and the O2 are different.
For the HE90, I like TF and Mullard ECC83, while for the O2 I prefer 5751 types. The TF and Mullard ECC83, and also Amperex Bugle Boy, provide a nice tonal balance and an overall very natural sound with the HE90. Very smooth and organic. Slow-paced, intimate jazz vocals and easy listening-type female vocals (aargh, I'm getting old!) are especially enjoyable.
The O2 benefits more from the 5751's crisper and cleaner sound. I think the O2 driven by the Aristaeus can be a bit dark-sounding and congested compared to the HE90 and HE60, or the 4070 and Airbow SR-SC1, so the hyper-clean and ultra-detailed Sylvania Gold 5751 is a welcome match for the O2. However, I don't think the Aristaeus is a bad match for the O2, because I prefer it over the KGSS I used to own as well as the Stax 727 and 007t amps.
Er, what was the question?
Ah, I think the ES-1 has better dynamics, more details and a larger soundstage than the Aristaeus. Every ES-1 is different, though, so I wouldn't assume my unit sounds the same as another (hopefully mine is above average at least
). With the ES-1, the HE90 is a more dynamic and more energetic headphones. Bass impact is stronger. The overall sound is more vivid. It rocks out more, as if it chugged down some Red Bull and a Viagra. With pop, rock and faster-paced music, the HE90+ES-1 combo is more involving and more fun.
I haven't tube-rolled the ES-1 much yet. So far, I've only used the 2C51 adapters and WE 396A, Tung-Sol and Raytheon tubes in the first gain stage, several 6SN7 types in the second gain stage, and a quad of Mullard xf2 or 3D21 tubes and adapters.
But with that said, I'm not sure if I would say the ES-1 is a better amp for the HE90 than the Aristaeus. The Aristaeus equals the ES-1 in refinement, my most important factor for comparison. It loses in dynamics, details, separation, and soundstage, but is basically equal in tonal balance and realism. And I think the Aristaeus is smoother-sounding, with more of that sense of music flowing easily like liquid. This is with some rare and costly ECC83 tubes, though.
With the O2Mk1 and O2Mk2, I think the ES-1 is significantly better than the Aristaeus. The ES-1's superior dynamics, details and soundstage benefits the O2 greatly, and makes it better-balanced and not slightly dark-sounding. The O2Mk1 in particular can seem a bit sleepy and sluggish driven by the Aristaeus, but the ES-1 wakes it up. The O2Mk2 is more forward-sounding and has more mid-bass impact than the O2Mk1, so I think it is less sleepy when driven by the Aristaeus and benefits less from the ES-1's dynamics. But it definitely does benefit. Both sound very good driven by the ES-1, with the O2Mk1 and O2Mk2 becoming my second and third favorite headphones, respectively.
My rankings with the Aristaeus: HE90, SR-Omega, O2Mk2, O2Mk1
My rankings with the ES-1: HE90, O2Mk1, O2Mk2, SR-Omega