So, I put a layer of copper (wired to ground) and a layer of mu metal ("MCF5" foil) around the signal wires running from the rca ins to first attenuator. And while I was at it I also put a U-shaped copper/MCF5 shield between the newly shielded signal wires and the OT-2 - it wraps around the sides of the OT, well away from any contacts, and is separated from the signal wire shielding by a cut-open piece of shrink tubing.
[WARNING: What follows are my purely subjective impressions and could very well be entirely imaginary and the product of self-suggestion -- in which case I say Hooray for the power of self-suggestion, because I like what I hear!]
I immediately noticed that music sounds "faster" and more energetic. There also seems to be some additional detail and clarity. No other changes noticed.
Pretty cool.
cheers,
Derek