Seeking Suggestions on Preamp Upgrade (CJ, Aesthetix, EAR)

FYI:
  The prevailing rule of thumb is that the input impedance of amplifiers should be 10 X greater than the output impedance of  preamplifiers.
  The Pass XA30.5 input impedance is spec'd at single ended: 15K Ohms and at balanced:  30K Ohms.
  c-j recommends an amplifier input impedance of 20K Ohms for all of the above mentioned c-j preamps. All c-j's are single ended only.
 The Aesthetix Calypso suggests recommended amplifier loads of single ended:  10K Ohms or greater and balanced:  20K Ohms or greater.
Couldn't find anything specifically re: the EAR 868, but Ear does suggest looking at the EAR 912 as this what the 868 what based upon. The EAR 912 output impedance is spec'd at:  600 Ohms.
 The Zesto Leto  output impedance is spec'd at:  150 Ohms both single ended and balanced.

What you need is to purchase a reasonable good preamp that response well to tube rolling. Most people use tubes to fine tune the degree of warmth.

EAR , Mapletree audio and Joule Electra are some of the tube pre that i find may have more warmth compared to CJ , ARC, Allnic and alikes.. do be aware that within its product line, there are also some difference in their sound characteristic too. PrimaLuna is another suggestion, however, personally i feel it's does not sound laid back, much better for music with singers. 

you have to judge for yourself how distant sound you feel that's right for you. There are many preamp that provide warmth but it's much more difficult to strike the right balance mix of detail, articulation and warmth that suited to your taste, hence tube and cable rolling sometimes can be quite handy. 
-phil

Make sure the impedance matches as suggested above by unsound, and weight your opinion about responses you receive toward those from users, as opposed to those that have not owned or used the equipment they are recommending or disparaging. 

My general opinion is that if I have not heard it I can't have a valid opinion on how it sounds.

David - there is a VAC Ren Signature on the A-Gon listing right now that would be perfect with your PASS and fit your budget easily as well.  Do NOT hesitate as this is a great pre at at great price (no affiliation, just a very happy VAC owner)

Unsound and jperry:

Thank you for the insight on the impendence matching between the source and load.  I read some on-line information on this topic.  It appears that the the 1 to 10 ratio is related to balance between the maximum power transfer and system power efficiency optimizationis.  Back to the actual impendence readings for each brand listed above, none of them can meet the requirement for the rule of thumb, c-j's output impendence is less than 500ohm, and barely 100ohm for ET5, interesting perspective.

thank you again.