All the Krell FPB (Full Power Balanced) amps are fully Class A to their rated power (which is 200 watts per channel on the FPB200). I'm not sure if there will be a big difference in slam/speed between these two amps as both are powerful and fast.
If I had to describe them, the Krell FPB may have a slightly creamy style of sound, but it is not slow at all - definitely has a lot of resolution and transparency and power.
The Pass Labs will have a tad bit more of a dryer sound from it's FET based input stage, but I would not call this a "dry sound" at all. It's just dry "in comparison to the slightly creamy Krell". The "X150" is not a Class A amp, and these have been noted to have more slam than the fully Class A "XA" series.
Both have very nice characteristics, but if you're listening to Klipsch with 6v6 amp, the Pass Labs sound will likely be closer to this but with slam and resolution.