What actually determines volume power? Is it watts?

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.

I have run into the same issue with my set-up.  I have a Parasound A21 amp that is rated at 250 watts at 8 ohms and 400 watts at 4 ohms.  My speakers are Martin Logan 60 XL, rated at 4 ohms and 92 sensitivity — so should be easily handled by my amp,  But I seem to run out of steam too at the higher volume levels.  On Reddit, a guy told me that amperage is more important than watts when it comes to volume.  He looked up stats for my amp and said his smaller amp had higher amperage and that is why he never ran out of volume on his rig.  I don’t know about these things, but I just assumed that a high wattage amp would be equipped to drive high sensitivity speakers to anything I could want.