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

I faced the same question, having found that adding a preamp always brought sound quality down a notch, compared to a simple analog volume control. My solution was the Schiit Saga preamp, which is passive if you want it to be, and has the option of adding a tube buffer should the impedances of your system be incompatible with a pure passive. The volume control is completely analog.  As an added plus, it has a remote control, and it was less than $400. The new versions, the Saga + and the Saga S, are tube buffered, or solid state buffered, respectively. As with the original, the buffer is optional and can be turned on or off with the remote.

Even cheaper than the Schiit Saga, but it’s just passive is the Schiit Sys $49. it’s 10kohm passive just like the Saga just not paying for the tube stage or remote. https://www.schiit.com/products/sys

Cheers George

I hooked my Fiio M15 up to the Fisher SA-100 and a pair of Altec 604B Hollywood's.  It was beautiful.  The buddy pushing the 400Cx-2 was not thinking there would be an improvement.  That is going in for a total refurb.  So we will A/B in a month.  But it sounds great as is.

The SA-100 has no volume control, so it is all via the DAC and works great.  

I was using the preamp on my Mytek Brooklyn DAC.  It worked well since it has an analogue volume control.  I had an old Joule Electra LA150 preamp.  I put new tubes in it and tested it to see if it could replace the preamp in the Brooklyn.  Admit to liking the sound with the JE150 better than the Brooklyn.  Was the sound better?  Not sure but the JE150 was clearly different and I liked that difference (more detailed).  So I would not spend money without testing first.  You might like the different sound ... but maybe not.

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

  As many have previously stated, if the DAC has volume control, no. Do you WANT a pre-amp? Probably,  yes. Worst case scenario, the preamp is distasteful. In such a case, sell the preamp. Carry on as you were, all the wiser.