At what point is an external D/A converter warranted or beneficial

I don't have analog to compare to, but I had similar issues when starting to build my current digital system - harsh and bright - and had the same starting point with the Oppo 103 transport/DAC.  I did pretty much what has been suggested above - first I added an external DAC (used PS Audio Digilink III) which gave me more detail but didn't help with brightness.  Then replaced that with PS Audio Perfect Wave DAC and Bridge server, which quickly left the Oppo gathering dust, again more detail and better coherence top to bottom, but still some harshness/brightness.  (For me, one advantage of the PWD/Bridge was that it sidestepped the digital cable question, as ethernet is used to feed the DAC via the Bridge.)  Next worked on power cords and outlet - and that really helped tame some of the brightness (ended up with Cardas Clear M for DAC).  Isolating the DAC did not do much if anything.  Minimal room treatments helped, but I imagine this doesn't apply to your case bc your analog SQ is plenty pleasing.  Tried different driver tubes on amp, which also helped a little bit, maybe.  Have not tried speaker cables but after reading comment above re AQ cable now I might. 

Bottom line for me was that it took a variety of things to minimize the brightness, but in terms of equipment, getting a somewhat up-to-date DAC was the single biggest improvement from an equipment standpoint, making me believe that continuing to improve the source should be my priority. 

Luckily (or not?), I don't have a nice TT etc to compare it to, and since I was committed to digital I could justify $2000-$3000 (similar to cost of decent TT setup) for digital equipment.  Also most of the music I enjoy is available through Tidal, so I haven't had to burn too many files over to NAS.  

gormdane and gillatgh,

     I have an Oppo 105 and use a similar configuration with my combo 2ch and ht system.  I connect all of the 105's outputs directly to my amps (center, sub and rear surrounds via rca cables and the mains via xlr cables) and the sq and convenience are very good.
     My only reservation is that the 105 was specifically built as a step up in audio quality from the 103 even though they're both identical in video quality.  I know the 105 uses expensive Saber dac chips on all channels and the 103 uses Cirrus Logic dac chips on all channels.  The 105 has no harshness or brightness issues but I'm not as familiar with the 103's sq. and hope it's to gillatgh's liking. 
     Considering Oppo's commitment to high quality, I tend to doubt they'd use the Cirrus Logic dac chips in the 103 if it resulted in bright or harsh audio.  
     Since gillatgh's Rogue Pharoah integrated combines a tubed preamp section containing two long-plate 12AU7 tubes combined with a Hypex OEM class D amp section, I find it unusual that this combination would produce a sound that would be described as anywhere close to bright or harsh.  I've also never read any comments on the 103's sound as having a bright character on either cd or Blu-ray playback.
     Overall, I still expect the system configuration I suggested to provide a well balanced sound but, worst case if some brightness remains, there are some other options left to explore.

Tim

@noble100, some things I've tried so far trying to isolate the issue. 
1. At Oppo 103 Swapped digital IC from optical AQ to coaxial AQ Haweye which resulted in an increase of high frequency brightness using HT Lexicon. No change noticed using the Rogue.
Note: connection to Rogue is via analog out of Oppo where the Oppo internal DAC is doing the conversion. There is some hf brightness but not as pronounced. I think the Rogues tube section is attenuating the hf signal.
Also noted that this hf anomaly is isolated to digital only not an issue during analog playback. Also streaming from Spotify or Pandora directly from Smart TV seems to be no issue. However when using the Pandora app in the Oppo the brightness returns. I have yet to get to trying your suggestion as that will take more effort and time. 
Ps. The Lexicon processor has never exhibited this phenomenon prior to the Oppo introduction. 

A point I forgot to mention, playing back a DVD thru the Oppo (concert, both BR and standard) exhibit none of the hf issues encountered when playing Redbook CDs. My issue seems to be limited to CD playback. Playing DTS cds also do not exhibit any hf issues.
Very strange!

Hello gillatgh,

     Yes, it is a bit strange but it's not brain surgery, right? 
     I no longer believe the Lexicon MC12B is the source of your issue.  It seems, in your current setup, you only use its internal dac chips for ht processing and you use the 103 for cd playback connected directly to the Rogue.

    However, I'd still suggest removing it because it's unnecessary and using the 103 for surround decoding, and connecting its outputs directly to your 5 ch amp, will likely be a significant improvement for ht sound quality and it'll streamline your system to just the necessary gear.
     Once you get the time to reconfigure and setup the 103 as the 5.1 processor, there are some other features on the 103 you can explore. 
     Things like the audio processing setup screens where you set the size ('small' for sub assistance or 'large' for no sub assistance) and relative volume of each of the 5.1 channels, the cutoff frequency for bass sent to your sub and how you want 2ch stereo cds to be processed and outputted.  You could get a head-start on this by reading these sections in your 103 manual. 
     I'll monitor this thread and await updates.

Tim