1. Assuming the same quality low impedence-ESR line of caps, why choose one amount of capacitance over another? Taking for granted there is a lower limit, isn't there also an upper limit - with capacitance discharge at larger values starting to affect rate of discharge and thus affecting peak signal timing/distortion? Maybe I'm completely misunderstanding this.

When using an SMPS together with a Class-D, two things are important, ESR and ripple-current. The capacitance value isn't that important at all. So having multible capacitors in parallel lowers total ESR and multiplies ripple-current handling.

When calculating the needed capacitance, they are mainly choosen for HF ripple.

2. Are there any acoustic properties at a set input level that would be affected by the capacitance you choose? I've seen reference to larger capacitance for more bass response or certain size of speaker driver...

You may do the math to see, that it wouldn't make a difference if choosen 470uF over i.e. 10000uF. It's still way to less to help in any way on your bass.

3. Are there ways to calculate what a recommended range would be?

Yes, lets do the math for your TDA7492 based amp. We will take the data from datasheet.

Po = 50W

RL = 6R

Vpeak = sqrt(Po * Rl) = sqrt(50W * 6R) = 17.32V

Ipeak = Vpeak / RL = 17.32V / 6R = 2.88A

Cbulk(min) = (Ipeak * Td * Dmax) / (Vripple)

with Td = 1/fsw (switching frequency) - lets take worst case minimum fs from DS = 250kHz

Dmax = maximum duty cycle, unknown so we assume 0.95

Vripple = dV = maximum voltage drop allowed at Ipeak, we assume 0.1V (even 1V wouldnt be an issue)

So:

Cbulk(min) = (2.88A * 1/250kHz * 0.95) / (0.1V) = 110uF

As we have two channels, we'll need at minimum 2x110uF = 220uF.

ESRmax = Vripple/Ipeak = 0.1V / 2.88A = 34.7mR

So either we have 2 caps with C = 110uF, ESR = 34.7mR or 1 cap with C = 220uF, ESR = 17.35mR.

This will be the minimum specs your bulk decoupling should have. When going for 400kHz its even less.

As the caps derate/ages with time, you'll choose your capacitance way higher, like twice so a 470uF <= 17.35mR would do.

The point for having those "big" capacitance values actually in place is, that the cap needs to handle Ipeak as ripple current while having that low ESR. As bigger capacitance usually have higher current handling and lower ESR those are choosen. But they wouldn't need that much capacitance.

4. When combining large wet and small polymer caps (popular oscons), if I understand it this is done to take advantage of the polymer treatment of high frequencies?

Mainly low ESR + higher current handling at 100+kHz.

5. Since polymers cycle much faster than wet and typically have lower capacitance, is there a calculation or ratio to determine equivalent capacitance between the 2, like 220uf polymer = 660uf wet? Or this a bad comparison because 1uF = 1uF; its the performance characteristics that are different. Is this important in a mixed parallel arrangement, or what if you want to replace a wet with a polymer?

See math above.

Edit:

To answer the question for "improved" bass response, lets have a look at an example. Picking some random techno/electronic music, a bass note is ~100ms. We now want to support the first half with capacitance.

There is:

C = (dI * dT) / dV

dI = current jump at full power pass, from the math above 2*2.88A (2 channel)

dT = length of current we need to support, 100ms/2 = 50ms

dV = voltage "sag" at current choosen, from the math above 0.1V

C = (5.76A * 50ms) / 0.1V = 2.88F = 2,880,000uF

Impractical?!

So if we relax the situation by choosing bass support for only the first 1/10 with a voltage drop of 10% of the supply voltage (Vsupply = 24V) we will still need:

C = (5.76A * 10ms) / 2.4V = 24000uF

Well, would do but whats for the rest of the 90ms? The supply voltage would drop down to Vsupply - Ipeak * Rsupply. If we assume a conservative resistance for the connection from the supply to the first cap of 200mR, There is: 24V - 5.76A * 200mR = 22.84V which is still higher that 24V - 10% drop = 21.6V. So this makes no practical sense. (to me)