With that chain (Hegel Viking → Cary SLP-05 → Plinius SB-301), I’d be careful not to chase the treble issue by simply “warming things up” indiscriminately — that can easily come at the expense of resolution and timing, which your components are actually very good at.
In my experience, forward or edgy treble is often less about “too much top end” and more about how energy is being transferred and reflected, especially between preamp and power amp.
A few things that have helped in similar systems:
1) XLR over RCA (if the Cary → Plinius pairing allows it properly)
Not because XLR is automatically “better”, but because the common-mode noise rejection and grounding behavior can noticeably calm upper-frequency glare in resolving solid-state amps like the Plinius.
2) Moderate conductor diameter, not oversized
I’ve consistently found that mid-gauge conductors sound more relaxed and natural up top than very thick, low-resistance designs, which can emphasize leading edges.
In practice, I’ve consistently preferred 0.35 mm or 0.55 mm conductors in situations like this — both tend to relax forward treble while keeping microdetail and harmonic texture intact.
3) Avoid heavily plated conductors and aggressive dielectrics
Silver-plated copper and some thick PTFE geometries can sound spectacular at first, but in already resolving systems they can push presence and air a bit too far forward.
A simple geometry, minimal dielectric mass, and good mechanical damping usually does more for treble naturalness than exotic layering.
4) Keep length sensible
I’ve had best results around 1.0–1.5 m for interconnects in similar setups. Very short runs can sometimes sound more “immediate” but also more forward.
If you’re happy with the system’s tonal balance overall and just want the treble to relax and integrate better, I’d focus on energy control and geometry, not on rolling off highs.
Curious whether you’re currently running XLR or RCA between the Cary and Plinius, and what length your interconnects are — that usually tells most of the story.