Preamp or not.

If you are getting the volume you need from your existing setup, then you don't need a preamp with any gain.  

The problem with controlling the volume from the DAC is that this is often done by reducing the number of bits going to the DAC, which reduces resolution.   An analog volume control doesn't do this.   Since you are using an external DAC with your Node 2i, the only way it can control the volume is to reduce it digitally before the DAC processes the signal.

I can recommend the Schiit Saga+ preamp. It is zero gain, and offers both a passive mode and tube buffer option. The volume is controlled by a stepped resistor network so the balance between channels is very precise. It also comes with a remote.  Cost is only $400 and they have a two week trial policy but do charge a 5% restocking fee if your return it.  If you need extra gain or want something fancier, the Schiit Freya+ preamp is $900. Both units are popular with lots of good reviews.

you do not need a preamp but a good one can improve the sound greatly.  
it is not so much about voltage but clean voltage and robust power supplies capable of fast changes and that greatly improve drive and dynamics.  
the analog section of DACs and digital players are what separate the good from the great and as so with the preamp feeding the power amp. 
look at the Rogue Audio RH-5 Linestage Headphone amp.  Review in Stereophile.   

https://forum.audiogon.com/discussions/high-end-stereo-preamps-worth-it?page=2

in particular, see mike lavigne’s post on second page... it covers well what is being asked

search bar above is always your friend... :)

If you can stretch a bit you could consider a Sachs pre that’d likely sound glorious with your wonderful amp.  Best of luck. 

I would consider a preamp with some tone control.that way you adjust some of the older recordings