does your master socket have the removable bottom half (accessable by 2 screws)? if so, you could try removing it and plugging into the hidden socket behind. this cuts all your internal wiring out of the equation. if the SNR margin improves significantly it could suggest bad local wiring or a noise source nearby messing with the signal.
simply remove the lower half and gently remove it and then plug the filter into the socket on the right hand side and then see what the figures are on your hub.
al