Which DAC as a preamp?

Bel Canto DACs- the 2.5, 2.7, 3.5, 3.7 all have 

built in preAmps with volume control. Bel Canto deserve more accolades, but they dont advertise much so are less well known.
they use proprietary super low distortion clocks, are accurate, musical, fast, balanced. 

I didnt see a budget listed for your DAC, but you can get a DAC 2.5 for a paltry $900, or a TOTL DAC 3.7 (which i have) for $2500. either way, I doubt you will ever look back. 

"Which DAC as a preamp?"

Go for one that has settable max analogue gain output, so you can use it’s digital volume control in the top 25% part of it’s range.
This way your sure of not "Bit Stripping". Just like Wadia and Mark Levinson do on their Flagship models.
Today I know M1 Bricasti has it, there must be others as well, these are the ones I would go with as they know about "bit stripping" that's why it’s introduced. As the Wadia attached shows.

https://ibb.co/d69hD9

Cheers George

I owned sources who has a pre amp inside. It worked well and there are more brands who use it. But.....when you use a pre amp, I have to admit that it is a lot more precise. Using a dac works, but to be honest it is not the best way to create the best.

I owned sources who has a pre amp inside.

All should get out of the habit of saying this. It should be "I have a source with digital domain volume control"

As "most" sources that have volume controls done in the digital domain DO NOT have preamps in them, they have standard low impedance output buffers. No preamplifiers!

Only if the specs of the source say that it can output more than say 6v to say 20v can you say it has a preamp which pre amplifies.

If it’s below 6v then it’s just a dac/cdp that happens to have a digital domain volume control with output buffer with or without some small amount of gain, to bring it up to Redbook standard of 2v some kick it a little higher 3 or 4v.

Cheers George

10 years ago I bought a Pass Labs XP-20 pre amp. After that I used my Meridian 800Daxv4 with inbuild volume control for a pre amp for many years. 

By adding a good pre amp you will hear the benefit of it directly. The precision of a pre amp is bigger than you can imagine. It gives both voices and instruments much more a shape. Beside that it creates better dynamics as well compared ot a Dac with an inbuild pre amp/volume control.

You need to learn people and also let them hear the differences by using a real pre amp. When you did not hear/audition it in a comparison it is not possible to understand it. There is still a lot unknown in the world of audio.