I've been running the KR 300B (straight bottle version) in my LIO DHT, and quite happy with the results, nothing 'wrong' with them.
Recently bought a pair of KR PX4. The first thing I noticed about the PX4 is it seems to have
a higher output compared to the KR 300B. I had to turn the volume down a couple notches to equal the sound level
produced by the 300Bs.
After matching levels, I still perceive the PX4 to have a "stronger" presentation, more presence or fleshed out if you will.
Tonally and spatially about the same as the 300B, in my system as least.
The bass seems to have slightly more power behind it, however that may be a function of the tube producing more output, I don't know.
I think I'll run the PX4 for a month, then put the 300Bs back in and see if anything's missing...
Interested in trying the TAKs, but I'm guessing the difference would be subtle, and not sure if it would be worth triple the cake.
However, If they did add gobs of depth, or separation, or soundstage width, I would deem them worth it.