1. No I haven’t had similar problems.
2. I don’t think it would help to buy a power conditioner.
3. Unless there is a problem with the Dac in your Oppo, there should be no benefit to adding another Dac unless the Dac is a better sounding one than the one in your Oppo, but that’s not what you’re trying to find out here.
I would not buy anymore gear until you figure out what the problem is.
I have not heard of anyone having problems connecting an Oppo to an ARC preamp but you have had the same problem with two different ARC preamps.
I would start by making sure your Oppo is working properly. Clean the contacts on the Oppo and the interconnects you are using. If the noise persists try a different pair of interconnects. Also try using a different input on the ARC.
If that doesn’t solve the problem, put your receiver or a borrowed preamp other than an ARC in your system, or take your Oppo to a friend’s place and try it in his system.
If you still have noise with your receiver or another preamp, the problem is in your Oppo.
If the Oppo is not making any noise with other preamps, then the problem is with the ARC preamp or the Oppo is not compatible with ARC preamps.
Try using a different cd player in your system with your ARC preamp. If you still have noise, the problem is the ARC preamp. If there is no noise with another cd player, then the problem is incompatibility between the Oppo and the ARC.
In that case, I would keep the ARC and find a different disc spinner to use with it. The Oppo is a good player but I think it is the weak link in your system as far as sound quality goes (just my opinion).