Amplifier "gain" question

if by "gain" you are referring to the "input sensitivity" of the amp to reach it's rated level (i.e. somthing like 1.5 volts, more or less) then the answer to your question is yes, except that the less powerful amp would clip earlier. Your latter observation is likely due to the current requirements of your speakers - all watts arn't equal (it might also just have a lower input sensitivity and thus seem to be more powerful.)

No. Gain means how much the voltage is amplified with respect to the input signal. Power means how much energy it takes to get it there based on the load an amp sees. As an example, if you were to lift an object, irrespective of weight, to a given height, that's equivalent to the gain. The power that you exert will be determined by the load you lift. So to lift a 10 lb weight and a 100 lb weight to the same height (at the same time - or else it's work, not power) results in the same gain but 10 times the power.

For an amplifier's power rating to have any meaning, it must be attached to a given load (100W into 8 ohms, 350W into 2 Ohms, etc.) Most amplifiers, regardless of power rating, will gain the voltage by about 25 to 30 dB. Most preamps will gain voltage by about 12 dB (notable exception: the CAT SL-1 with 26 dB, which is why they're great with SET amps and low efficiency speakers). Given the same load (speaker impedance and efficiency), the amp with the higher power rating will give the same gain with a lower volume setting. The power output for both amps will be the same, which is why the speakers will give the same "loudness", albeit at different volume settings.