KEF PSW2010 and NAD 3225pe interference or faulty?

[DistrictLuke]

Hello All, long time reader first time poster! I'm having an issue in my setup that so far I haven't seen documented elsewhere so i'm hoping someone can shed some light!

List of equipment

Dual 505-3 Turntable
NAD 3225pe amp
KEF PSW2010 Sub
Monitor Audio BX1 Bookshelf speakers

Problem

I am using the sub as part of a 2:1 setup for my hi-fi system. The amp i'm using an early 90's NAD 3225pe amp. This has a link between the pre and power amp section allowing you to use it as pre amp only or an integrated amp. This is where I have inserted the KEF sub from the NAD preamp into the low level input of the KEF and then the high level output of the KEF back into the power section of the NAD. Reason behind this is because I am limited to bookshelf speaker size ( i'm using Monitor Audio BX1's ) and the KEF filters out frequency below 130hz (or 100hz not 100% sure ) before it goes back into the NAD which then powers the speakers.
This setup actually works really well as the speakers aren't trying to recreate the same notes as the sub and it makes the mids and highs much clearer and It sounds great apart from a high pitched whine thats present.
The whine itself is approx 5 khz in frequency and very low volume, you can only hear it when its in a silent part of music or the volume to the rest of the system is turned down. It isn't present if you remove the KEF sub from the circuit and It doesn't change with the volume on either the amp or the sub, i'm wondering if it is a failing component inside the sub that is causing this to happen?

Hoping someone on here will be able to help!

Many thanks

Luke

 

[Chris Frost]

At 5kHz you'd hear that in the main channels without the sub in the loop, and so if the whine goes when the sub is removed then it's either a fault in the sub or the cables are picking up some interference. How old is the sub?

 

[DistrictLuke]

At 5kHz you'd hear that in the main channels without the sub in the loop, and so if the whine goes when the sub is removed then it's either a fault in the sub or the cables are picking up some interference. How old is the sub?

Well the sub is new to me, but I’m gonna guess it’s probably circa 2003. It has got a label on it as being repaired in 2017 but the issue that was repaired is unknown to me.
The whine has only been present since introducing the sub and if I remove it from the system then it goes away so I can rule out the amp and speakers from fault.
It’s annoying as the setup works and achieves exactly what I want it to but this whine drives me crazy!