Need cable to soften brightness just a little?

I would suggest you replace the receiver with a suitable power amp that would mate with your preamp. 

@dill   Good Call!👍

Yes, good call @dill .
  I missed the fact that a vintage receiver is driving the system. I'm confused as to where the Arcam and the tube amp play a role in this system.

In any event, that Pioneer is a beauty, but not up to the task of providing the high quality sound you're looking for.   Just for kicks, can you install the Arcam and remove the Pioneer? The Arcam is 90 watts and Class G, so it has a very efficient power supply.
Blue Jean cables will reveal the true sonics of a component, they are very unforgiving. This test will show if you can achieve less fatiguing sound.

I have compared the Arcam and the Pioneer.  All I have to do for a/b comparison is to mute one or the other.   The Pioneer leaves the Arcam in the dust.  The only difference is the Arcam is getting audio via HDMI, the Musical Fidelity/Pioneer by way of RCA interconnects.

History:  The tube amp, the Audiomat Arpege, had for some time been a regular alternate to the Arcam SR250.    The Arcam was used for regular tv viewing, the Arpege for musical programming.  One set of speakers on each.  HDMI cables to the Arcam, RCA audio  interconnects between the Arpege and all video sources.   Why?  Because the Arcam simply didn’t sound that great in comparison.  In spite of  Arcam’s reputation for good sound, it’s still an AV Receiver.  Believe me, considering how much it cost, I’ve wanted to like it better.

I was recently  inspired to try my old Pioneer in place of the Arpege.  The Pioneer was long sitting idle as the phono stage for a seldom to never used turntable.  It occurred to me that it would be much better suited to the TV system than the Arpege.  So I swapped around some gear.  The Arpege went to the computer audio system where it’s an ideal match with the small Spendor S3/5s there.  And then another idea,  why not use the  Musical Fidelity that was there at the computer, which had more than enough sources for the many pieces of gear in the TV room, plus a handy remote control, something  much missed in the TV room!  Even better, paired with the solid state Pioneer I no longer will feel I have to ration listening for fear of burning too many hours on tubes!  Darn tubes are costly and I can’t change them myself which is darned inconvenient.

Result:  This change is orders of magnitude better for audio and for dialogue than the Arpege and both leave the Arcam in the dust.  Even my husband, no musical aficionado by a long shot, provided an unsolicited comment about how good it sounds.

  I can easily switch between the Arcam and the  Pioneer.  The Arcam sounds somewhat muddy and somewhat veiled in comparison. The Pioneer, all 120 watts, sounds lively and clear and musical and I am loving the sound of that extra horsepower which is lurking in reserve.  The Arcam is now merely a switching device, albeit a painfully overpriced one.  I can even do away with the second set of speakers, as all sources are connected to both the Arcam and the Musical Fidelity/Pioneer.

The test I really want to make is to temporarily remove the Musical Fidelity and try the Pioneer alone.  Remember, I am bypassing the Preamp on the Pioneer. I don’t know how much the Musical Fidelity is contributing.

Brightness (edginess) is inherent in many CD's. Restore your Pioneer receiver to full functionality and use the treble control to adjust the high frequencies to your liking!