+1 for the Backert Labs Rhythm 1.3 with NOS RCA 12SN7 GTA. Replaced my ARC Reference 6 SE with it and couldn’t be happier.

Cary Audio SLP 05.  get a super deal during their holiday sale. 

All potentially good choices.   I recommended the Zesto because it has everything under the hood.  RCA , XLR,  2 pair RCA out, two pair XLR out , transformer coupled inputs and outputs.  Unbelievable sound,  super quiet, dynamic , detailed ....

Output impedance is 100 Ohms.   Great preamp, even better company.  Zesto support is first class.  I can't say enough good things about them. 

The Atma-Sphere has the output impedance you require, which needs to be at least ten times less than the amps input impedance. Too bad it's a boutique company though:-) CJ has been around a very long time. The two pres I owned from them had outputs of 50 ohms though,so nope.Best of luck in your search.

Atma-Sphere MP-3

See below:

Mike

http://www.atma-sphere.com/en/mp-3.html

+1 Rocray. At one point, Cary audio was a small outfit, too. 

CJ makes very nice ones 3, 5 i think, SE series. used ones are a steal

I would suggest the Zesto Leto Ultra II preamp.   I had the original Leto with the ESP power supply upgrade and it sounded amazing     The latest model is even better.   

I would suggest the Backert Labs Rhythm 1.3. Fully balanced and the best preamp I’ve owned to date. When you start tube rolling, get some adapters and use NOS 12SN7’s to replace the 12AU7’s. It’s very sensitive to tube rolling.

Might take a look at the Bryston BP 17 cube. balanced impedance is 4.5kΩ. If you step up to the BR 20 you get their DAC module (which you already have) and streaming platform.  There is Also a Modwright LS300 over on TMR for $11K.  Don't know anything about Modwright myself but I do like running Tube pre's with SS amps.

What preamp are you using now, and what kind of sound characteristics/improvements are you looking for?

Thanks for the recommendation, rocray. I understand that in this industry, there are a lot of great designers producing great products on a bespoke / small-scale.

I’d prefer to stick to the more established lines if possible.

I would recommend reaching out to Aric Audio. He now offers his Motherlode II in a fully balanced model. The balanced model is the Motherlode XL.  http://www.aricaudio.com/products.php?product=MLXL#desc No affiliation, just a very happy customer. I own the non balanced version,the Motherlode II.
Aric is very easy to get in touch with, and is very knowledgeable.