Preamps w/ polypropylene caps in power supply?

Those film caps can and do sound great compared to electros in many designs. But it depends on the power supply design, design goals, and type of amp. I have used computer grade, huge can electos to great effect. They do great things for a robust and kick butt power supply. Those film caps just don't offer large enough values for some designs.

Poly power supplies have many advantages with the biggest disadvantages being size and cost. However, when modifying a product's power supply over to all or even partial poly from lyrics is not as simple as replacing caps. Whenever you switch caps, you run the risk of altering the voltages due to difference in resistance. You always have to check voltages of the stock power supply before touching anything. You will likely have to modify dropping resistors to assure you end up with the same voltages as before you start. There are even times where you will not be able to obtain the proper voltage range. Sometimes you will end with voltages with the range accepted but this is rare depending on how extreme the modification is.
These different voltages are often the cause for the changes in sound.

We use polypropylene power supplies in all but the Econo versions of our Musica Bella products. Again, this is due to size and cost. This topic has been brought up several times and is becoming more popular. I have always finished with saying you have o pay attention to both the power supply and the circuit design. You can have the best circuit design in the world but it means nothing without a great power supply (poly or lytic) and visa versa. It the product as a whole that yields the best overall design. Unfortunately, most manufacturers usually concentrate more on the circuit design than the power supply but this seems to be changing more over the past couple years.

Dielectric Absorption.
Charge up an electrolytic capacitor and then time how long it takes for it to discharge to zero using a bleed resistor. Remove the resistor, wait a minute and read it again, there's more voltage left to come out. This is an example of dielectric absorption. It's like squeezing a sponge dry, and then some more seems to come out.

A polypropylene capacitor discharges faster and more completely than an electrolytic. In addition to electrolytic capacitors, some power supply designs use a big polypropylene cap (similar to start caps used on motors) to make reserve power more immediately available.