Using a variable with array of ElementIds to use in craft.entries.id
I'm not sure where you're getting the id's from but you will need to pass an array into craft.entries.id so try:

{% set navsubids = craft.amNav.getNavRaw("hoofdnavigatie", {startFromId:25})  %}

{% set navsubidarray = [] %}

{% for navsubid in navsubids %}
  {% set navsubidarray = navsubidarray|merge([navsubid.elementId]) %}
{% endfor %}

{% for entry in craft.entries.id( navsubidarray ) %}
  {{ entry.title }}&lt;br&gt;
{% endfor %}


When you used {% set navsubidarray = navsubidarray|join(', ') %} you were creating a string of the values in the array.