Power amplifier to drive pumps
You actually need +/24V @ 4A if that was suggested.  But it all depends on your acceleration and speed demand. 50W gives 1/2 torque and RPM.

To do this, depending on your pump acceleration, you may need a reserve capacity > 100W or not.  Typically motors use 5 to 10x rated current on full voltage start. So a ramp start is better.

The best solution for power is a 24V battery with a trickle charger because of the cranking current capacity or low ESR with a cheap 20A PWM motor speed controller with gain.

The cheapest solution is a universal 24V 4A laptop charger. https://www.banggood.com/AC-100V-240V-DC-24V-4A-96W-Power-Supply-Charger-Converter-Adapter-p-970663.html?rmmds=search&amp;cur_warehouse=CN with a PWM motor speed control board rated for 5x the current like 10 or 20A. for $10 with fan when needed.

Otherwise Audio amps are usually stereo so 100W is actually 50+50 and you would need a 200W stereo or 100W mono unless you had 2 such systems to control.  The cheapest cost here is $0.15/W from Banggood in 50+50W systems.  That means your pump only goes half max speed if rated for 24V but still works.

100W cheap https://www.banggood.com/AC-100V-240V-DC-24V-4A-96W-Power-Supply-Charger-Converter-Adapter-p-970663.html?rmmds=search&amp;cur_warehouse=CN  but not as reliable so get 2.

20A driver $50 https://www.ebay.com/i/123582962532?chn=ps&amp;dispItem=1

If you wanted digital control, there are many with USB interface.  This one is more than you need https://www.robotshop.com/ca/en/sabertooth-dual-2x32a-6v-24v-regenerative-motor-driver.html?gclid=Cj0KCQiAmuHhBRD0ARIsAFWyPwj8goYFI9DrjU_M9EIaslp7uu4dwMlnrOQVLryy0MSJgf4Ef8NBaWMaAoelEALw_wcB

Cheapest 43Apk 28V motor driver https://www.banggood.com/Semiconductor-BTS7960B-43A-HBridge-Motor-Driver-Module-For-Arduino-p-928634.html?rmmds=search&amp;cur_warehouse=CN