I was using a tempest (non-extreme)pre for my mono extreme.
rest of gear: Slinkylink silver cables, IC, benchmark DAC1.
I found the sound very clear, deep base, detailed highs,a nd very good value for money.
I late changed to a Modwright SWL9.0 SE
I found the improvement in soundstage dept, 3 dimensionality of the sound, depth of bass and high extension quite significant. The instrumental separation is superb and music , especially mass orchestral pieces, does not becomes congested when things gets complicated. And, it doesn't have that slight SS edge, and the sound is so smooth yet detailed.
It also cost 2x the Tempest, but I love it!