"the best amp" thats a tricky question. a better question though "whats the best amp for what im looking to get out of my type-x's". everyone's situation is different.
also remember the enclosure is going to have a huge impact on how much power they will want. you may find that a specific ported enclosure and around 500w rms to each might be perfect for your listening needs, and your electrical abilities. or you may need more. car audio is definitely not "black and white". its grey region is vast indeed.
because that is what kind of enclosure they are geared towards.. they dont sound as good in ported boxes and they will have a peaked responce slightly above the tuning. so say you have a box tuned at 32 hz.. at around 38 hz or so is where the sub will play the loudest and on both sides of the EQ curve it will drop steeply so the frequencies will be unbalanced