symfony2 collection set default values - php

Question

Using Symfony2, we have a Product form which has a collection of productOption forms. This works fine, however we wish to have some values within the productOption form set to 0 by default.
I have tried solutions given in other StackOverflow questions, but I still just get blank fields. Currently, the desired default values are being set via the Entity's constructor as such:
// ProductOptionValue Entity
public function __construct()
{
$this->option_values = new \Doctrine\Common\Collections\ArrayCollection();
$this->subtract = true;
$this->price = 0.00;
}
The prototype of the form collection is not standard - it's modified via a macro within the template which outputs the form:
{% macro product_option_value_prototype(product_option_value) %}
<td>
<div class="product_option_value_widget">{{ form_widget(product_option_value.option_value) }}</div>
<div class="product_option_value_error">{{ form_errors(product_option_value.option_value) }}</div>
</td>
<td>
<div class="product_option_value_widget">{{ form_widget(product_option_value.quantity) }}</div>
<div class="product_option_value_error">{{ form_errors(product_option_value.quantity) }}</div>
</td>
<td>
<div class="product_option_value_widget">{{ form_widget(product_option_value.subtract) }}</div>
<div class="product_option_value_error">{{ form_errors(product_option_value.subtract) }}</div>
</td>
<td>
<div class="product_option_value_widget">{{ form_widget(product_option_value.price_multiplier) }}</div>
<div class="product_option_value_error">{{ form_errors(product_option_value.price_multiplier) }}</div>
</td>
<td>
<div class="product_option_value_widget">{{ form_widget(product_option_value.taxed_price) }}</div>
<div class="product_option_value_error">{{ form_errors(product_option_value.taxed_price) }}</div>
</td>
{% endmacro %}
... Which, in the same template is used via:
<table class="product_option_values" data-prototype="{{ _self.product_option_value_prototype(form.product_option_values.vars.prototype)|e }}">
I have tried setting other values as the defaults (10 instead of 0) in case Symfony ignored them at 0.
I have also tried implementing this CollectionTypeExtension Gist. Whatever I try, we still get blank fields by default. Because other solutions on StackOverflow seem to give people success, I feel there must be something wrong elsewhere to the entity or FormType, but we are having no success in finding it. Any ideas?