If I just use a DAC, do I need a Pre-amp?

As mentioned above, the Schitt SYS passive preamp has analog input for two sources and cost $49 plus a few bucks shipping.  I have one and like all Schitt products its a great little unit.

I've been looking at upgrading my speaker cables.  So, was playing around with my system setup last week.  I've an Audio Research Pre and Power amp and a DCS Bartok DAC.  

I listened to this with the AR Pre in and out of the system.  Getting the volume equal was a bit of work.  The most obvious difference with the preamp in vs out was with the presentation of the soundstage.  It was much better defined and spacious, with the preamp in. 

So, if you have a DAC that has volume control, no you don't need a preamp. Depending on your system, you may find that a preamp adds a lot!   

One DAC, has an analog potentiometer, the other from the software player.

These are the specs of the DAC I plan to use.  https://www.fiio.com/m15

As posted above - tried and liked already.  I may try a pre-amp in a month to see if I like the sound color.  Mainly because as a vintage one, I expect I can sell it for what I paid and then some.  I would not buy a new pre-amp even if "better" as I'm more likely to not get my money out of it.

Subject = Ad Nauseam........My Luminous Audio Axiom, Walker mod, 3 in, 2 out, with remote, is just what the doctor ordered. I do not hear it, in my signal chain. The chassis does not resonate, the connectors and wires are top notch, the switch and control, are also, top notch. It works with my 3 dacs, 4 tuners, and my many ( still ) amplifiers, and is, ime, and imo, perfect....for me, and, that, is all that matters. Enjoy, and stay safe.....Always, MrD.

If you have the availability of software attenuation, i.e. ROONs media playback software offers the option, then you will want to compare use of the software volume control versus the hardware volume control. 

Results will vary on performance, and there is no telling prior which will be superior. I always compare the two regardless of which integrated DAC is being used. Results are not universal; with one integrated DAC the software vol. control yields a superior result, while a different integrated DAC I use yields an opposite result, and is far better with the hardware vol. control and the software output fixed. You simply must try them both. 

Beware that if you are not careful, a software volume control can be bumped, touched and send the system into max output. Caution is necessary when using such attenuation. However, dedicated DAC direct to amp is always cleaner, if not fuller. YMMV 

I strongly recommend the addition of SONORE's systemOptique to convert Etherent signal to optical, as it drops the noise carried on Ethernet. Striking change, improvement. See my article about it at Dagogo.com  

I have no interest in arguing with people about "bits" and resolution, etc. Superior performance is easy to hear.