"I'm considering replacing them with maybe some used speakers around $2.5k (pair) like some B&W 804s."
You'll make the problem worse doing that. Also, you're not going to solve the problem by just going out and buying something new. The whole system contributes to the problem you are having. You need to have some type of plan or goal as to where you need to end up, or you'll spend a lot of money and not fix anything.
Looking at your system, I suspect the problem may be shared between the tweeter in your speakers and you preamp. But just remember, anyone who hasn't listened to your system first hand, is guessing. You can start by connecting your CD player directly to your amp and use the volume control that Oppo gives you. Play some CD's that are giving you the most problems, and see if things get better, or worse. That will give you an idea as to what you need to do.
You'll make the problem worse doing that. Also, you're not going to solve the problem by just going out and buying something new. The whole system contributes to the problem you are having. You need to have some type of plan or goal as to where you need to end up, or you'll spend a lot of money and not fix anything.
Looking at your system, I suspect the problem may be shared between the tweeter in your speakers and you preamp. But just remember, anyone who hasn't listened to your system first hand, is guessing. You can start by connecting your CD player directly to your amp and use the volume control that Oppo gives you. Play some CD's that are giving you the most problems, and see if things get better, or worse. That will give you an idea as to what you need to do.