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

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. 

The old purest will hook from a phono directly to a power amp with gain control, that's it...

That's actually the one source which absolutely requires a preamplifier (and its fixed EQ).  Phono cartridges are akin to microphones or guitar pickups.  They are transducers whose output is so low that there is far too little gain (as well as mismatched impedance) to feed a power amp.  There is also a matter of applying the RIAA EQ curve.  In order to get the most out of the medium, extreme EQ is applied to the master.  That EQ is applied in reverse in the phono preamp.  Phono inputs on a preamp are different than any of the other inputs.  You should never plug a line level signal in to a phono input and never plug a turntable into any input *but* the dedicated phono stage.

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.