Rectification, whether it be via tube, or diode (silicon in most cases), is what causes "tightness" in an amp the most, I would say.
I could be wrong, but AFAIK this is the explanation:
Tubes do not respond as quickly to voltage as a a solid state diode does, so the amplifier "sags" with tubes. Diodes respond so quickly that the amp remains tight.
The quality and winding of the output transformer of the amp, which is the final stage in turning the signal inside the amp into something that can be run into the speaker, also plays a huge roll.
The "focusedness" of the circuit matters a lot. In general, an amp that has high q values for its EQ pots will make more of a "defined" sound. The attenuation or amplification of the frequencies will be more precise.