Should I eliminate my preamp?

In this particular case, its a bit of apples and oranges.

The ARC is single-ended, but the Oppo has balanced capability as does the amp.

The simple fact is that balanced often sounds better because of improved cable immunity, and that the amp might sound better driven by a balanced source.

The ARC OTOH is getting a bit long in the tooth- and is certainly old enough to have filter caps failing as well as tubes.

In this case, I'd be all about dumping the preamp until its serviced out, but that is not to say that an active balanced preamp would not be able to help. Right now there are a few too many variables to be scientific about it.

This is exactly what I recently did. I bought an Oppo Sonica DAC and asked myself "what do I need a preamp for?" Like you, I have an ARC pre-amp (SP-6B) but since I no longer listing to vinyl, I was just using it as a volume control. The Sonica DAC has a perfectly good digital volume control which sounds fine to me. Plus, the Sonica DAC has balanced XLR outputs, and my Rane AC22B crossover has balanced inputs. I noticed a marked improvement in quiet when getting rid of the SP6B and it's single-ended interconnects, and moving the the DAC/Crossover/balanced XLR setup. How could an extra piece of gear in the signal path improve anything?

My SP-6B recently came back from service at ARC, and I'm am wondering if I should sell it. There are a lot of new vinyl fans out there who might want it (along with my SOTA Sapphire turntable, Linn ITOK II tonearm, and the Dynavector Karat 23R cartridge).

No doubt, you have to listen to your system with and without and decide which sounds better...and in my own system, no doubt at all, much better with the preamp than Oppo direct to the power amp..in the mean time, just for fun, check out this video...

https://www.youtube.com/watch?v=Dh27E7YKN9s

It all depends on the preamp!

Happy Listening.

However, since the OP has all the tools to do the test for himself, he is best to answer his own question.

+1
I have always found that if you have enough gain for the loudness level you want or need, then going direct (with no preamp) has always been the most transparent, dynamic, least coloured way of listening to the source.
So long as the source is not a tube output, which can be high impedance, then you may get a impedance mismatch to your amp.

Nelson Pass made a preamp he loved called the Aleph L, which is passive until the volume control get’s to 3 o’clock, after which an active stage come in if you need more level, but as he says at a detriment.

Nelson Pass: " At the 3 o’clock volume control position, the Aleph L offers a direct path from input to output.
The only component in the signal path is wire and switch contacts.
At positions below 3 o’clock, the volume control functions as a precision passive attenuator using discrete resistor ladders.
Above 3 o’clock, active gain is added to the output signal in 2 decibel increments, for amaximum of 10 dB.
As a result, you suffer the effects of active circuitry only when additional gain is necessary."

Also a Nelson Pass quote:
" We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.

Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.

Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.

What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.

And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp."

Cheers George