CDP or DAC for $4000 budget

Hi friends,
I intend to upgrade all of my system, speakers will be Marten Bird.
My taste is neutral, soft, treble should not be so high, bass soft or enough.
Could you please suggest some CDP or DAC (used) for $4000 budget? By the way, please suggest an amp to run Marten Bird.
As I study from the Internet, there are some candidates:
1. MBL 1511 version D + 1521
2. Emm labs CDSA SE
3. Metronome CD 1 (Tube CDP)
4. Einstein The Last Record Player
5. Audio Aero Prestige Signature
6. Audio Aero capitole ref se
Thank you so much!
Do you plan on listening to anything before you buy or are you just pick something at random? If yes, can you list all the brands that you have access to, and the rest of your system?
I would avoid Audio Aero because you can't get service for them if needed from what I have heard from several sources.

REGA ISIS Valve cdp (used) .....

it also has the USB input that permits you to use its internal hi-end DAC to play all your external stored digital files with full remote features.

= both for the price of 1 .
Audio Research Reference CD7 can be had for about $4k.
It is one serious tube cd player.
Dear Guys,
Be honestly I haven't try any of them before. As you can see, I am building up a new system matching with Speaker Marten Bird and my taste is natural sound, not colorful.
@ Akg_ca: Rega Isis seams to be around $7000 which is voer my budget.
@ Mitch4t: Do you think Audio Research Reference CD7 is a right choice?
Some people recommend me for Weiss DAC202 and Berkerley alpha series 2.
What do you think?
Silversong, for $4k, I'm sure you are going to find an excellent solution. My only experience among the items listed is with the CD7, and I think it is a winner. I paid $9k for mine when new and I believe I got a bargain. If your preamp is up to snuff, the CD7 will not disappoint.