New to Preamp - Preamp suggestion around 10k

Ear-Yoshino 912, Atma-Sphere MP-3 (or MP-1 used).

I’m going to second the ARC Ref 6SE. They come up used once in a while at $12k ish….

Conrad Johnson ET5 or ET7 

That sounds like an unusual arrangement for a Stax amp. Bypass its internal preamp with an external one? There is no such preamp stage in most direct-drive Stax amps, which need to supply 50 - 60 dB net gain (more similar to phono stage, minus the EQ). I guess you could bypass the volume control, or perhaps you’re dealing with a design that puts traditional pre & power amp stages before some output step-up transformers to supply the needed gain.

Anyways, you can’t go wrong with the ARC Reference 6 or 6SE. They’re fantastic. Or a VAC Signature (used) if you want more midrange sweetness. I’ve tested both (well, a VAC Master in my case) feeding headphone amps, and yep they sound great. I also had an RP-9, and it has a leaner midrange with less sweetness than the ARC and VAC. Though I did also try the RP-9 feeding a headphone amp, and it can indeed work well too - nice punchy bass. The Ref 6 and VAC have a lower noise floor than RP-9. There are reasons they cost more.

Also, quite honestly I prefer Rogue’s older Athena and Hera models over the 9, but I understand I’m a bit of an oddball ;)

I currently have a VAC Master, ARC Ref 6, and VAC Renaissance III in my stable and I STILL hang onto an Athena because I just like it and it’s so good for the money (I regretted selling my Hera). It's dirt cheap compared to these other preamps, when it does pop up for sale.