Volume level of ARC Ref 5 seems very high

The Euphrodites is a high-very high eff loudspeaker @ 98db/watt/meter, which means 1 watt @ 3 meters would give you 92 dB, so it's not surprising that you have to stay near the bottom of the volume attenuator's range. It is also true that at the low end, many attenuators do not perform very well (no idea particularly about the ARC). You may want to consider reducing the overall gain of the pre. Some older ARC pre-amps had an internal jumper that would reduce the gain by half, or sometimes it involved de-soldering a component. Given the quality of the unit, I'd talk to ARC about changing it, unless you want to go to a lower powered amp.

Have you checked the gain level on the Ref5?

The Ref5 does not use a mechanical attenuator. It is microprocessor controlled.

I do agree that the gain on the Ref5/5SE is very high. And yes it is a challenge pairing with high gain amps and speakers that are efficient

I tested my Plinius SA-Reference with my Thiel CS3.7s in my room and while the Plinius was able to drive the speakers well, I thought the ARC wasn't in its optimal range (about 30+ on the display with the 5SE and 40+ with the 5).

So I swapped back to my Electrocompaniet Nemos to allow the ARC to work in its more optimal range.