ARC ls 25 mkii cd/dac input too hot?

I have an Audio Research ls25 mk2 preamp and pass x250.5 amplifier. I have a Bryston bda-1 dac and Acurus acd11 CD player. When I listen to CDs and the recorded music gets louder, it sounds like the signal going into the preamp gets too hot creating a little distortion. I’ve tried a few different CD players, both connected directly to the preamp, as well as using them as transports through the dac. Both created the same issue. I don’t think I’ve noticed it when using the iMac as a source for the dac.... I’ll have to check that again.
I’ve had no problems when listening to records. The signal is definitely lower.

It seems like the output of the digital sources is too loud for the preamp. Does that make sense?
Have you tried adjusting the gain on your preamp? There are 3 different settings per input.
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Andirocks, yes. I have them at the lowest setting. 
Are you running balance from your Bryston to the ARC? If you are, try a single ended cable to see if that helps. You mentioned running  a couple CDP’s directly to the ARC so this probably isn’t the issue as I’m guessing they don’t have balanced outputs. If you are using balance on the Bryston try single ended.

Also if you’re running balance to your Pass, try single ended as well.

I had a ARC LS25 MKII at home for a short time on my Pass X250 and had no issues whatsoever and the older X250 has a higher gain output than the .5 series I believe. Have you had your tubes checked lately?
According to the schematic the LOW/MED/HIGH gain setting is applied between the selection relays and the volume control chips. If it's still too loud on LOW that's a weird combination of levels.

Is the BAL/SE switch in the right position? It's for setting input configuration, not output.

Good luck. With a 2 volt source MED or LOW usually work fine.