New to Preamp - Preamp suggestion around 10k

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.

Zesto Leto Ultra is a fantastic preamp 

Take a hard look at the SST Ambrosia. World Class pre with excellent phono and headphone amp as well as single ended and fully balanced connections.  Competes with about anything out there and can be found used on occasion