I hate to rain on the electromagnetic parade, but I am connected via TOSLink Optical cable
Hmm - that is the best way from the electro-magnetic interference point of view. But it doesn't exclude the DC lead from the mains block as a possible source (if
interference is the problem in this case).
My preference would be to try the clip-on type like these...
- try part number N95AB
The solid type that you thread the cable through may be marginally better but as long as the two halves go together without a gap then there's not much to choose (and you can move them, use them elsewhere, etc). If possible, loop the cable through more than once since that will increase the effectiveness. It has to be said that Maplin are not cheap, but at least they are accessible.
See also here for another source...