I’ve owned Vandersteen 1B speakers for a long time.They are easy to drive and have a 6 ohm impedance.These are not suitable for an SET. Generally speaking, to get the best out of any SET the amplifier should never be driven past about 20-25% of full power. If you do, higher ordered harmonics begin to appear, at first only on transients. The ear uses those harmonics to sense sound pressure, so this causes the amp to sound very 'dynamic' but its really distortion masquerading as dynamics. If you've read about how dynamic SETs can be, this is why. A speaker at least 10dB more efficient that the Vandersteens is needed; even the Coincidents are a bit on the low side, notwithstanding the fact that they also sell SETs.
One way to deal with this is to get a sub or subs (the best being the Audiokinesis Swarm) and then get a speaker that only goes down to 70Hz or so. If the SET does not have to make the bass and the speaker doesn't either, the choices available goes up quite a lot!