Wrong analysis.
1. If you have two amplifiers, both rated to drive 8 ohms, with different maximum power ratings, and you connect them one at a time to an 8 ohm speaker, you can get more output from the speaker when driven at higher power.
2. If you adjust the gain (volume control) to get the same power (within the maximum limit of both amplifiers), you will get the same output from the speaker.
3. You do not "double the volume" with twice the power. Sound level is perceived logarithmically, and doubling the volume is not a defined concept. See the many discussions of the Weber-Fechner Law, e.g.,
https://ocw.upc.edu/webs/42254/Acustica_EN/Bloc2/Fitxes/T07_02_Llei_Weber_Fechner.htm4. If amplifier 1 is not loud enough to annoy the neighbors, then amplifier 2 will give you more sound output. (Not the result you stated.)
5. The speaker rating of 45 watts continuous/100 watts peak is a maximum rating: you may damage the unit if you apply more power. The amplifier ratings are the maximum output with a specified level of distortion.
6. The voltages you mention for the amplifiers are not correct: 19 V rms into 8 ohms is 45 W mean power (NOT rms power, a common misnomer) and 24 V rms into 8 ohms is 72 W mean power.
7. The speaker sensitivity rating is a specification of how much acoustical output you get for a given electrical input. It is defined as the indicated sound level (in dB SPL) obtained 1 meter from the speaker with 1 W electrical input. The definition of "dB SPL" is the ratio of the acoustic power divided by a very low reference level, usually 20 x 10
-6 pascal sound pressure, which is roughly the threshold of hearing, and defined in the ANSI S1.1-2013 spec for sound level meters.