I think you may have run across some IRS that AK'er have little experience with. The older IRS series (Beta, Gamma, Delta) as well as the Ren series have had much discussion and a few of us own them (one member has the real IRS if I remember correctly), but I haven't read much here about the Sigmas.
I own Gammas, a friend had Betas, I've heard Inspiribomb's Ren series. All of them are fantastic and are bettered by few. With that quality of speaker you generally end up finding personal preferences and strengths rather than weaknesses.
I'm sure the Sigmas perform very well. If my memory serves me, they may have been one of the final efforts to keep the IRS line going after Arnie Nudel left? So without Arnie's guidance they may still have the IRS magic or may not. Someone correct me if I'm wrong, but I'm too lazy to do any googling at this point.
They are not true dipoles that most of the IRS (they do have a rear-fire emit I think), and do not have the (in my opinion) magical L-emim. Instead they use a mid-bass coupler similar to the Ren 90s. No servo control on the bass. I would guess they sound like 'big' Ren 90s? There's no replacement for displacement! I've never auditioned the big Kappas, so I can't comment on those.
Buy them, use them, then tell us!
