Benefits of an external DAC


I need a sounding board from you folks.  I have a slightly upgraded Oppo 103 CD player that was a big SQ improvement over my Consonance tube CD player.  I need a DAC to take the stream from my iMac, turn into an analogue signal to my preamp.  Lots of folks rave about the benefits of an external DAC so, in progression, I got a Jolida tube DAC, a Schiit Gungnir, and a Channel Island Audio DAC.  I connect the Oppo to the CIA DAC with a coaxial cable.  I input the analogue signal from the Oppo into my preamp with RCA IC's.    

It is easy to switch between DAC in the path vs. the Oppo analogue signal straight into the preamp.  I tell ya, I have done the comparison between all three DAC's vs. the analogue signal run from the Oppo to the preamp and have been unable to discern any difference.   Zero, zip, nada.  I have done the comparison with lots of audio pals and nobody hears a wits worth of difference.

There clearly are lots of devotees using external DAC, but in my system (with a Don Sachs tube preamp, Pass Labs First Watt F5, and Spatial Audio M4 Turbo S speakers, with good cabling), I am perplexed as to why the use of an external DAC makes no difference in the SQ in my system. It sounds exactly the same.  

I am awaiting a demo of a Denafrips Ares DAC to see if this well-regarded DAC does something to the SQ -- one way or another.  So far, the employment of a DAC to improve the SQ of the signal from my Oppo has been an utter waste of time.  What am I missing?
whitestix
I have a 103 D with a NAD M51 and love it....i had DACs years ago before hi rez audio and always enjoyed having a digital hub for lack of a better word.    I have everything digital connected to it including a blutooth receiver to stream via my tab.   Newer dacs may have internal blutooth or wifi not sure but ive always been a big proponent of outboard dacs.....   my first was an Audio Alchemy and then i quickly upgraded to a Cal Audio Labs Alpha tube dac and ive had several since.   I had a CIA VDA 2 which was a killer dac, awesome for the money......
Helomech,
I guess my system is about $10K, and is the most revealing ensemble of gear I have ever had... and I have gone through a boatload of electronics and speaker in the past 20 years to arrive at this magical set-up.  I can honestly live very happily with the sound of my Oppo player.  "Cheap and cheerful", that is the way I roll these days.  
Oddiofyl,
I have the CIA VDA 2 with an external power supply and the comparison to the signal straight from the Oppo into my preamp vs. through the DAC provides no perceptible difference in SQ.  Maybe I have connectivity issues.  I will report back to you folks how I fair with the Denafrips home demo.  
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@shadorne. I don't really know where Stereophile ranked my two DACs, the Mytek Manhatten and Bryston BDA-3.  I imagine that since they reviewed each DAC very favorably that they have them high on the pecking order.
  Regarding cost, the Manhatten was going for 5K new (mine was a demo so I paid less) and the Bryston was $3.5 K.  I've written on this before so I hope that I'm not being redundant but I purchased the Bryston specifically because it has HDMI.  I own a lot of SACD and Blu Ray Audio and wanted to play the DSD and DTS streams in my two channel system.  I thought that I would be selling the Mytek but found I can't live without it!  If only it had HDMI it would have been the perfect DAC.  The Mytek is much more transparent than the Bryston on Redbook or when accepting digital feeds from my Bluesound Vault2, but the Bryston is no slouch with Redbook and has enhanced my SACDs.  The Bryston certainly bests the Oppo with both Redbook and SACD, and I lived very happily with the Oppo as a DAC for 2 years or so.
  So based on comparing the Bryston, the Oppo, and the Manhatten, my experience confirms that as one ascends towards the $5K mark in DACs they just keep getting better.  I wish that I could go all the way until dcs territory but I've hit my limit