I have compared Spires directly with Summits on the same wall with the same gear. I always have loved Summits and never had any complaints about them in any way. What is really interesting is that the Spires make the Summits sound like its' bass colors the midrange a bit. I'm not saying that Summits sound colored or muddy but hearing them after hearing the Spires made me feel that way.
When I say 'cleanest sounding' I guess the best way I can describe that is a direct comparison to the CLX. The CLX just sounds to free of anything that could potentially get in the way of the music. The Spires a similar kind of quality. The sound is in my opinion more like a finely tuned Summit with less bass. Their new crossover is brilliant and I will be very curious to hear the Summit X.
These speakers are crushing my room. The bass gets a bit out of control when I start cranking but at a medium volume they sound fantastic. I have a bit more work to do with placement but that will come after the panels are broken in. In the meantime I will be enjoying these along with a nice Maker's on the rocks.