Recent sound deterioration shrillness, ringing

Thank you. I'll try to address each of your comments together.

1. I've been using the Behringer for well over a year so to whatever extent it degrades the sound, it becomes relative because the system has always sounded fine until now.
2. When I spoke with Peachtree they said the only way to hook up a sub is to use a splitter from the pre out to deliver the signal simultaneously to the powered sub and the power amp. That works but it substantially degrades overall sound to the extent that I described earlier. Also, separating the signal like that gives the EQ no control of low bass needed for auto room correction.
3. I spoke with Behringer who ok'd the use of the unbalanced aux out stereo connection for the sub. This works fine and I assume that this will accommodate auto room correction as well.

Bottom line is that it looks like this setup works but I still have this problem with degraded sound. I'll be back on it tomorrow and will report what I find.

Thanks, Kijanki, even though I only sort of understand your explanation as it might apply to my situation your reference to switching inspired me, for some reason to look look at my interconnects again as possible culprits. This may be because, being a new user of XLR cabling, phase issues have been in my thinking. Now, I realize that, even though connections in and out of the equalizer are balanced, because connections to from the preamp to the EQ and from the EQ to the amplifier, are through RCA adapters, balanced signals are not being delivered. Anyway, because I tried everything else and determined that the problem had to be between the preamp and the equalizer somewhere, I decided to check the interconnects.

I removed them from the system to check for simple continuity and found something that can only called strange. Although the RCA cabling all checked out, when checking the resistance between the pins on the XLR's I found a VERY low resistance reading between 2 of the pins on one of them. In checking between the same pins on the other cables there is no evidence of even the slightest continuity. This seems impossible; in my understanding and experience, there is either a short or not, not just sort of a short.

Anyway, I just happened to have an extra XLR cable which I installed and everything sounds great again. I went one step further and reinstalled the one with the anomaly and the crummy sound returned. So...I found the problem but am totally baffled. It even crossed my mind that there might be an impedance issue but the voltage applied to the cable by the meter is DC and because this anomaly didn't present in any of the other cables, this could not be the issue.

What I found here seems so unlikely or even impossible, I'm keeping the cable for others to evaluate. BTW, the cable is relatively new showing no damage of any kind.