The only reason i feel the need to respond to this, is that i hope people in the future don't look at it as truth.
The dB unit is a form of measurement. 100dB is 100dB no matter the source, just as 65mph is 65mph regardless of the car. Also, if you are wearing open headphones in a quiet environment at 50% volume and they are at 85dB, then you are putting 85dB into your ears. Now you do the exact same thing outside in the city, and find it is harder to hear, your are probably still at 85dB. Turning up the volume so your can hear will only increase the dB (volume of sound/noise) to your ear.
Also, bigger speakers and open headphones does not automatically mean you need lots of power. How much power you need depends on how efficient the speakers are, the source, and the acoustics of the environment. For example, if you try to play some well mastered classical music, it is often done with very low gain to preserve as much detail as possible. Likewise most EDM, rock/metal, and especially pop tend to ramp up the gain in mastering all the way to the upper limits. What this means is what while you may only need 25% volume for most EDM/rock/pop, you may need 75% to achieve the same dB of volume. Note: this has nothing to do with what format it is in (mp3 or lossless), and everything to do with the actual studio that made the music. Point is quiet source = more power for same dB of volume. This can further be taken to the discussion of preamps, but i digress.
Next, the efficiency of the speakers plays a major part. I've owned old passive bookshelves with 6" woofers, 12" woofers, and modern towers with multiple 6"+ woofers. I've also owned closed and open headphones. The point is that I've had large open headphones that were easier to drive than some smaller closed backed ones. I've had giant tower speakers that were easier to drive than smaller single 6" bookshelves. What matters is the efficiency of the drivers themselves.
Lastly, acoustics. People vastly overestimate the actual power required to drive speakers, especially when you are only 4feet away from them as you say you are. It doesn't take more than 15 Watts to get 100+ dB from most speakers with music like metal when you are so close. Hell, it doesn't take more than 15 Watts to drive them to loud volumes in a large master bedroom. As the room gets larger, and you move further away from the speakers though, more power is exponentially required to drive them.
So to counter what your said simply: no, you do not need "big power" to get the best from large speakers, and no it doesn't matter whether the speaker is an iem, open back headphone, or giant tower speaker, 100dB is still 100dB to your ear.