for example roland cube 15 and cube 30...do they have the samw tonality?
yes and no.

if you look at a 40 watt fender amp, and a 100 marshall stack, they will obviously sound different. of course the wattage difference wouldnt really be much a a reason why they sound so different.

but if you had two amps that were the same, but one was 100 watts and the other was 50 watts, (many amps are like this, for some reason i cant think of one of my head, jcm800 maybe?) there would be a difference in tone. this is largely due to the different sizes of the transformers in the amp. so its not really the wattage difference that changes the tone, but what has to be changed in the amp to get more watts that matters.

edit. i didnt see your second post before i posted. i hope this helps. and edit your post like this instead of double posting.
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SS amps work in a few different ways and so there's not a straightforward answer for them, but tube amps need to be designed differently for different wattages, so yes. You can't just add or subtract wattage arbitrarily without changing other variables, and those things predictably change the tone of the amp.
i believe wattage does effect a speaker's response to low end, in the fact that it takes a speaker more energy to make a lower note at the same decibel rating. this has something to do with with it being less efficient to produce slower oscillations while maintaining amplitude of the oscillation. but this has more to do with matching an amplifier to speaker configuration for desired frequency response(like the 1x15 won't give the high end of a 4x10, while the 4x10 can still produce low frequency because of it's effective speaker area when taken as a whole). this phenomenon is more of an indirect correlation of wattage to tone, but it is still relevant.

i think you are more likely to see a result of tonal difference because of the components and how they react at different volume setting rather than the simple 'more/less' wattage observation. like a 15 watt solid state amp will sound more different from a 100 watt half stack more because their difference of components and technology than because of their rating difference.

that being said, there are times when high wattage amps are desireable because of their attributes, like when you need tons of stable headroom for a drop tuned metal band. 100 watt amps will be able to support your low end more while keeping power amp distortion at more of a minimum, this will help keep your tone more stable and avoid the much hated 'mud'.
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