How Do Amps Affect Soundstage?

I'm not that technically strong on audio yet, so please refrain from mockery on this....

My DAC, premamp, and amp combo (all tube) throw a nice soundstage.  If I substitute (at least some) solid state stereo amps, soundstage is constricted.  If the amp is basically just increasing the signal that it is receiving from the preamp, I don't get how the size and shape of the presentation is altered materially from what the preamp is delivering. (I get that the signal could get distorted, etc.).  How does the amp play such role?  And do monoblocks enjoy any design advantage in maintaining the soundstage received?  Thanks.