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

A DAC with a volume control built in may be just fine, but then again, it may sound better running through a preamp. It depends on the "match" with the power amp you are using. I run a BelCanto DAC/Pre straight into a pair of Bel Canto mono amps with excellent sound, but they are designed to work together without the need for a preamp. Try before you buy if  you can.

to my ears, preamps impact the system's tone, speed, and resolution. to me and in my system, they have had more dramatic impact than amps (that's not to say that amps and other changes haven't). i picked up a nice soulution pre and am never looking back. i've done the dac with volume control but it's not for me.

Yes you can, but the impedances of the equipment might not match, causing anomalies.  

Yes you can, but the impedances of the equipment might not match, causing anomalies

There is no impedance match problem with today's solid state sources, but there "can" be with boutique "tube output sources".

Cheers George 

Have tried the following preamp configurations with my Wilson Alexia 2s, Linn Selekt DSM with Katalyst DAC, and Pass Labs X600.8 monos:

1) VTL 6.5 Signature II [tube-SS hybrid] ($18K)
2) McIntosh C2700 [tube] ($8K)
3) Audio Research Reference 6SE [tube] ($17K)
4) NO PREAMP - direct to amp

The Audio Research Ref 6SE for this particular set up was the winner, with all options having unique characteristics that were quite good unto themselves.

But we aim for a natural, warmer sound that encourages long listening sessions and deals well with a broad and eclectic spectrum of musical genres. This translates into us favoring a tube preamp since our amps are solid state.

The direct connect from the Linn Selekt DSM to the amps sounded lethargic just enough that after a few songs we were done listening. Sure, the highs were crystal clear and the bass had punch. But there was inferior cohesiveness between the highs-mids-lows, and the highs were a bit tinny and borderline fatiguing while the bass was not as tight in relation to the rest of the musical frequencies—compared to the tube-based Ref 6SE.

Throwing the Ref 6SE back into the line-up, the musicality returned and we listened for hours, sampling every type of music. We never returned to the direct DSM-to-amp config.

Since we have Pass Labs amps, we are getting in a demo of the XP-32 preamp, which we have heard good and not so good about. But we are checking out the synergy that may arise from both pieces (meaning the preamp and amp, that is, as the XP-32 is a 3 chassis set up and the monos are, well, a 2 piece deal) being from the same manufacturer.

My point: YOU have to listen for yourself. Never let anyone tell you what you will find best. It is YOUR room, YOUR system, YOUR ears and YOUR music. All the electrical theory and bits and bytes mean nothing if you hate it. This is art more than science. You are a living soul, not a robot.