I'd come to realise there's definitely something different re: the core tone of the 'old' WB-100 and the new mid-knob version (and presumably this applies re: the R-400 too).
The new version seems bigger, wider and cleaner. I chatted to Leszek about this, because I quite liked the more complex mids of the earlier version, which just 'happened' with the bass and treble set flat.
His answer was simple and obvious - 'Turn the mid knob up more boldly'.
To paraphrase the longer answer, the preamp in the new version is built so the mids and highs saturate less readily, to achieve a super-clean signal.
So, last night, I did just that. I pushed the mid knob (600) more than I ever normally would on an amp, rolled the bass back a little... and introduced the mid glow I was looking for. Yum.