What actually determines volume power? Is it watts?

@immatthewj Really depends on zooming in on what the OP is hearing when he feels he's run out of juice. 

Generally speaking, a current constrained amplifier produces a flat output into an easy load but at difficult loads the frequency response starts to track the speaker's impedance curve somewhat. 

So with an ESL, an amp will lose output in the treble.  Roger Sanders of Sanders Speakers rightfully points out you need an amp with not just current, but high frequency current drive, which his Magtech amps provide.  With a speaker with a normally bad bass output like several Focals, the amp will sound soft in the mid-bass, where the speaker dips to or below 3 Ohms. 

Agree that we need to know the speaker.  In addition to amplifier power, speakers for the consumer market may start to compress early on.  That is, the power output is no longer proportional to input due to physical limitations of the device, including thermal.

If you double the power, and go from 1 W to 2W you should see +3 DB across all frequencies, but most speakers quickly start to compress, and produce less than 3dB gain per doubling of power. 

If you are at home, and 100W is not enough, you should evaluate the amp+speakers and even the room before making a fix, but jumping in amplifier size is not likely your answer.

Just as a matter of curiosity...,

My Yamaha RX-Z9’s menu allows each input a separate, "trim" volume control so that you can match the apparent volumes from the different sources without resorting to the primary volume knob each time you change the input source.

I’m assuming that your Yamaha would have similar (maybe a bad assumption), but if it does, have you tried to turn up the volume here so that you don’t end up at 70% of the main volume knob setting?

I’ve definitely found that some albums stream louder than others.  I really hate playlists for that reason. Two songs play at a good level and then the next song comes out blasting so loud it makes your ears bleed. Than the next tune is quiet so you turn the volume up.

Cl a ss d amps like d sonic pascal ant vtv ncore can put > 2000 watts into 4 ohms and sound much better that previous versions.there around 2k$.a fosi audio mono with 48 volt gan power supply from fosi is about 200$ and puts out 300 watts mono block. Most have 30 day trial. Power or watts is energy times amps that's the equasion.class d amps are light 80% efficient due to design. Marantz mcintosh denon ect are now all making class d amps. Might be worth a try for you.does your yamaha have a pre out if so use it as your pre amp.enjoy the musicand the search.