I have the same speakers B&W 801 Matrix S3 and I am also a Threshold lover. I currently have the T2 modified by Jon Soderberg and a pair of T400 bridged to monoblocks. The sound is warm and musical with a lot of dynamics. One thing that you need to be careful matching a Threshold amplifier with another brand pre-amp is with the pre-amp's impedance output spceially if you are going to use the balanced connections. Threshold amps requierd a pre-amp with less than 100 ohms in the impedance output on XLR to work fine otherwise you will have somebuzzing noise coming out of the speakers (I speake by own experience).
I had the S405e before. It is a great amp and will run the 801s fine but if you really want them to sing their best I would recommend you to move up to a S550e at least or even better a pair of S12e or what I did a pair of T400 bridged at monoblocks.
I had the S405e before. It is a great amp and will run the 801s fine but if you really want them to sing their best I would recommend you to move up to a S550e at least or even better a pair of S12e or what I did a pair of T400 bridged at monoblocks.