Hi guys, so I hooked up the sub after initially presuming this would exacerbate the current issues.

It's just a tiny REL T3 but works well in small rooms just to underpin the main speakers, nothing earth shaking.

Took a while to dial the correct setting with roll off and volume level but I've managed to get somewhere close to no boom. It still occurs on acoustic basslines (guitar, double bass etc) but only on certain tracks and at a touch above average listening levels. I can't actually see the back of the sub as its tucked around the corner behind my vinyl but I think the phase is set to 180 so I'm guessing this is having a cancelling effect to some of the standing waves and room modes.

It's really letting the midrange of the quads shine through again and I reckon with a few more strategically placed pillows/books along the back and side walls I could well achieve zero boom.

I may also move the sub around a bit to see if the cancellation effect can be improved.

Thanks for all the suggestion and help I'm pretty humbled so many were willing to give their opinions and advice.

Will update with any further findings anyway.

Cheers,

Chris