In my application, I attached a complete CV for every reference, so it may seem less critical to have good letterheads if the adjudicator has access to such information. The letters themselves should explicitly mention how the reference is qualified to evaluate your abilities and accomplishments vis a vis the national interest.
If the choice is between an impressive letterhead and a letter containing exactly the magic words "national interest" etc., then I would go for the latter. (Thus, my issue is not so much that the letters are addressed to "To Whom It May Concern", but that they aren't explicit enough in addressing your national interest waiver case.) The letters have to be crystal clear in arguing that you serve the national interest; in my letters, for instance, my references addressed quite explicitly the listed requirements for NIW.
Besides, how bad are their current organizations anyway? Their current letterheads should be ok. Do remember to attach a CV with every letter.