The all-tube is wonderful - it just sounds bigger than the R-400. I was actually saving to buy it, but someone beat me to it (lucky for them!).
In the meantime, I'm back with the R-400 and I'm in love with it, but the touch-sensitive draw of the WB-100 will probably win eventually.
Truth is, I would (and have been) smitten with both, which I guess is a credit to the preamp.