New to Preamp - Preamp suggestion around 10k

I would recommend a used Audio Research Ref 5SE or REF 6. The current model is Ref 6SE which is nearly $20K… very worth the money. I have owned many. It REF 6SE is the center of my system. But from your description this is likely best fit. Or Conrad Johnson or VAC… these are the top tier and well worth it. Used is fine they are built to last decades. 

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.