I literally am getting custom silicone sleeves from 1964EARS for the Flux. Yes, I know that paying $100 for a $50 IEM seems crazy, but I really do like the Flux's sound. (Plus, I hadn't heard of the Klipsch oval tips fitting when I ordered them. )
That said, 1964EARS have taken sleeves off their website since then (I ordered them in late December), so you'd have to go somewhere else for that option.
I only have medium Klipsch gels. I feel like the Klipsch gels run a little smaller than the stock SS tips on average but I don't have the other Klipsch gels to confirm.
The Spark is more v-shaped, with less deep bass and treble extension and mids that are not as rich and full.
That’s a little crazy, alright . Hope they sound even better with a custom fit!
RE: the EPH-100 – it sounds to me like you’re after something leaner. The EPH-100 has a ton of bass, a pretty warm tone, and a unique treble character with the energy coming in pretty high up in the spectrum. It’s a quicker driver compared to the GR99 so it doesn’t have the VSonics’ issues, but considering that you’ve gravitated towards the CC51 and SHE3580 (aside from presentation) I think the tonal balance and thickness of the EPH-100 might be problematic for you.
P.S. Yes, those came to mind because they are in your budget
Honestly I'm not convinced the A81 is as good as the GR07 quite yet. Jury's still out on it though. I would say Phonak 232 with Comply tips but that costs quite a lot more and doesn't have more forward vocals. The Alclair Reference would be even better but its bass is more in line with the GR07 than the GR07BE.