Voltage Regulator for Amplifier
For starters, I'd buy a premade module based on the TPA3116 chip. This gives you a subwoofer channel and a left and right channel. A good reason to buy one premade is that it's difficult to design the filters with feedback in the class D amp. Also it's cheap.

You can add your own bass/treble circuit in front of the module. Also, you could replace the potmeters (which you should) with smd resistors and add a volume circuit before the module if needed. So that the class D board has the largest possible gain. I noticed that the potmeters on these modules are of below average quality and can introduce noise in the signal.

As for the powersupply. Something like this will work, maybe look for one with a bit more amps if you want to be safe. With this powersupply you can also supply your own circuits with a simple regulator since your input voltage to your circuit will be 24V.

5A should be enough for only driving the subwoofer. If you use the 2 main channels, I'd buy one with at least 240W (24VDC 10A).

As for the calculation of Justme in the comments:

P = V*I = V^2 / R = R * I^2 (by Ohm's law).

P is fixed (P=100W) and R is fixed (R=4/omega), with this you can calculate both V and I. While keeping the ratio V \ I = R = 4