So what's going on here? How is a single 0.1uf cap between a DAC and the ampifier input doing this?

Here is a simple way to visualise the effect of a capacitor which forms a filter.

Note that for this explanation I'm simplifying things and ignoring things like phase angles.

Ok here we go....

A capacitor of fixed value will exhibit a varying impedance (think AC resistance) to different frequencies.

This impedance is called capacitive reactance (Xc) and the relationship between Xc and frequency as follows:

So lets assume the capacitor is in seriess between the output of a DAC and an input of an amplifier and lets

call the output impedance of the DAC Z1, the reactance of the cap Z2 and the input impedance of the amp Z3,

the simplified equivalent circuit as follows:

So we can see that the cap (Z2) is in series with the DAC (Z1) and if you plug different values into the formula above,

you will see that the value of Z2 will increase as frequency decreases.

This total value of Z1+Z2 will form a voltage divider with Z3 and as frequency decreases, the value of Z1+Z2 will increase and create a greater and greater loss as frequency decreases.

You have just formed a high pass filter.

Now let's take an example where the cap is between the DAC output and ground:

(again Z1 = DAC, Z2 = cap and Z3 = amp)

Now since Z2 (Xc of the cap) increases with lower frequencies, the reciprical is also true, i.e. it decreases as frequency increases

and therefore Z2 will create a lower and lower impedance (to ground) with respect to Z1 and attenuate the high frequencies more.

This is the basis of a low pass filter.

Keep in mind however that this is a simple 2 pole filter and thus the attenuation versus frequency in both cases will be very gradual.

To increase this rate (or slope) one would use many stages of filtering and also use capacitors and inductors to further increase the slope.

Again I repeat that this is an over-simplification and I have ignored things like phase delays, pass band ripple and so on but I hope it serves to show exactly what the capacitor between the DAC and amp is doing depending on if it's in series or parallel with the two.