"I was talking about an SS amp of equivalent power rating"
A few factors come into play here I believe. One is that Tube Amps have a much lower Damping factor than a SS Amp. This can produce a fuller, more noticable Bass responce in some speakers with the same power. While I generally believe DF over 50 - 100 make little difference, from 5 - 100 might very well.
Tube Amps often have a dedicated 4 ohm Tap for the speakers. While many SS Amps will work into 4 ohms, there not optimized for it like the Tube Amp to make it more stable into a speaker like this. SS Amp's can oscilate and self distruct in short order under this condition. It's not just about Current requirements.
Also Tube Amps Soft Clip, Permitting Them to Play perhaps 3 db Louder than their SS counterparts (although I don't recomend trying it with home systems), and up to 6 db or more in commercial applications if necessary, where clipping the SS Amp would quickly fry the speakers.
Just thinking out loud, I use a SS Amp myself, but researched Tube Amps before I bought it. It could simply be that the Tube Amp is a better Amp, despite the difference in power output. Power is the last thing I look for in a quality Amp. IMO 25 - 50 wpc should bring most speakers, including the Kappa 9's to very satifying SPL's if they can handle this complex a load, and your pretty close to that.