Help. System sounds thin and bright or harsh

Several have mentioned if you place the speakers close to the front wall you will decrease the depth of your soundstage.

I have found that to not be true in my dedicated listening room with acoustic treatment all across the front wall, side walls, rear wall and ceilings.

Plus, there is no TV between the speakers (remember it's a dedicated listening room), and no rack of gear between the speakers.

My speakers are B&W 803 D3s with the backs of them 8" from the front wall and get a wonderful 3D soundstage if the recording is done well.

Schitt Audio has a few inexpensive passive EQ devices.  I use Roon, which conveniently has a built-in DSP function.  Guaranteed to ameliorate audiophile angst and make listening a lot more fun.  😁 

A more costly, but effective device is the DSpeaker Anti-Mode 2.0 which has gotten high marks from Robert Greene @ The Absolute Sound, and others.

There are no perfect audio components, rooms, recordings, or ears.

Okay….couple thoughts.  The move closer to the wall is giving you a bit of gain which is common.  You get about 3dB from the wall.  Move into the corner and it will jump to about 6dB.  But you will lose refinement. 
 

I am not seeing good measurements of those speakers but I do believe Harbeth s are best when toed in toward the seating position.  What you can’t assume is that you are getting a softer, less edgy sound because you are off axis.  I have a set of speakers that I make where they absolutely brightest when they are toed in about 10%.  When you toe them into the seating position they border on dark.  I am not saying it will be best fully toed but you should try it.

you have a tough room from a setup perspective.  You can do a few things.  You can experiment with treatments both to soften the treble and to enhance the bass.  What would be most interesting would be to get a full range microphone and measure at your seating position the frequency response.  This will let you see what the room is doing to the frequency response.

finally, there is nothing obviously causing the edginess you are experiencing.  I do run into this when there is a mismatch in terms of gain and impedance between components.  Too little or too much gain can or too weak or too strong of a signal from any component leads to that hard edginess.  
 

might want to try a preamp with adjustable inputs and outputs.  

My guess is that there is a setting somewhere in the preamp or Oppo that is completely not obvious and takes diving into several settings/screens to get to that was accidentally set.

Just wondering, you don't think this has anything to do with the modding of the Oppo?

One other thing….you don’t have any of your cables coiled do you?  Depending on insulation levels, you can get capacitance or inductance when cooling that can negatively impact performance. This can be a huge issue with ribbon style cables.