Yes, indeed, power amps can give you a "bigger soundstage" and improve dynamic range (both at low levels and high ones). They can also have profound effects on timbre, resolution, image focus, and--most important of all--the pleasure you take in listening to your music.
And, no, more power is not necessarily better power. I just recently heard, at Robert Harley's house, a pair of 17W Lamm ML2.2 SET monoblock amplifiers make Magico Q7s sound, well, magical. I also heard a 350W Rowland monoblock make these same speakers sound marvelous. Each amplifier brought a different set of virtues to the presentation (although there was plenty of common ground between them, too).
Now, you wouldn't want to use the Lamm ML2.2 with, oh, an mbl 101 E Mk II or a Magnepan 20.7—both of which have very low sensitivity. Indeed, the sensitivity of your loudspeaker, its impedance plot, the relative complexity of its crossover, the size of your room, and the kind of music you listen to (and the levels that you typically listen at) will determine the power minims you need.
But power itself isn’t anything like the whole story. Amplifiers spec’d at the same number of watts can be as different sounding as loudspeakers themselves are, depending on the design and implementation of their gain strategies. You have to decide what kind of listener you are—what aspects of the music you love (e.g., timbral beauty, performance detail, in-the-room-with-you presence, dynamic liveliness, low-end impact, midband realism, etc.) give you the greatest pleasure--and then you have to find the amp (and source and preamp, BTW) that delivers these things to your satisfaction.
The beauty of hi-fi is that beyond certain givens (such as having sufficient power to reliably drive the load that your loudspeaker presents to your amp to the levels that you want to achieve in the room in which you’re listening), there aren’t any hard and fast rules. You decide what sounds best to you.
Wow, all that informatiom and I'm not closer to solving my quest. So really it's just interchanging gear until you find the right gear, that matches your other gear, that gives you the sound you are looking for in your room. So, it's just a matter of saying I'm satisfied/ I give up !
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