Help with crossover capacitors

I get that, but does that actually deliver more wattage to the speaker? Or does it give headroom to the circuit?

The resistor power rating is the maximum amount of power that the resistor can withstand. It won't give your speaker more power, I believe the designer use a 15watt resistor for a reason.

Paralleling caps to make up the correct value sounds better than a single cap as long as they are equally the same quality. Have a look at Dueland you can get those from Hi Fi Collective.

Chaps,
When you parallel caps and resistors, you also change the inductance, which may or may not affect the sound depending on the rest of the circuit.

danybad:
I would remove the shelving control and associated resistors to make the system flat , get a good air or ferrite core inductor, improve layout so the cap is not in the inductor magnetic field and anchor the parts to they don't flap about in the breeze.

Be advised that it is impossible to evaluate a capacitor [-or anything else-]  in isolation. A cap that is a bit zippy with a ringy tweeter may be dulcet with a soft dome.

FWIW, I used Dayton pp to replace mylars on ancient Audax silk dome tweets and they are stellar.