Should I eliminate my preamp?

You may think at first that you're hearing a more transparent signal but a preamp properly matches the impedance between stages and provides more muscle (better dynamics) in the long run if it's a fairly good pre which it sounds like you have. You will get more dynamics and you won't be as susceptible to roll-off of high frequencies. The pre-amp is more than just a handy switching device, it provides an easy load for whatever you plug into it and outputs a lower impedance which will make the amplifier happier than if you just plug in a source device.

The first truly high end piece of gear I bought was a pre-amp. I couldn’t believe that change it made in my system. Without changing my midfi CD player or power amp, I was hearing a 3 dimensional sound stage for the first time, and a less brittle, much warmer and rounder sound. It excited me so much I rushed to purchase a high end power amp. It added bass authority and an even rounder more musical sound, but the soundstage magic was still a product of the preamp. I’ve been a preamp believer ever since, and would not consider using only a DAC with a volume control. That’s my limited experience. 

I most certanly prefer my Audio Research LS27 to no preamplifier by far.  In fact if I play my 35 year old or so Stax headphone system thru the record out of mt LS27 instead of the audio source directly in the audio reproduction is extremely better thru the tube LS27.  It is not just a minor improvement in sound quality.  Whatever audio source is used, say even my OPPO 205 as well as 105, the addition of a fairly recent Audio Research tube preamp will be the best choice.  By doing so a quality close to40 year old Stax headphone system can actually compete favorably to some of the recent expensive planer magnetic headphones of today.  For a fraction of the cost.  That is what a true quality tube preampkifier can do for an audio system.  It is simply someting one has to try out for themself.  Recently Paul McGowan of PS Audio said the same on his own current top audio systems.

As a long time (>20 year) user of and believer in passive line stages / removing active line stages, the answer is a big "it depends."   I.e., it depends on the output impedance of your source, the overall quality of the output stage (including both volume control mechanism and current delivery) of your source, the capacitance of your interconnects, the input impedance and sensitivity of your amplifier, and (of lesser importance) the sensitivity of your speakers.  Beyond that, I don't see enough information about your components and system to give a more definite recommendation.  I own an older Oppo (BDP-83se), but don't believe it has output volume control; however, my general impression is that I don't think the output stage is of the quality that I would necessarily prefer a direct drive sound.  By contrast, some of the aftermarket Oppos that significantly upgrade the output stage are suitable candidates for direct drive to the amplifier.

It's always been said by audio designers as a gauge of how good a preamp is since the mid 1900's, when separates (preamp & poweramp)  entered into audio, that saying is:
  
"The best preamp should sound like a piece of wire"
  
ie: a like a direct source to amp connection.  


Cheers George