Confusion on how to get ticket from Amsterdam Central to Berlin with German Rail Pass

I don't know why you seem so focused on 1st class, when 2nd class is perfectly fine.

However, right now you can get a 1st class Savings Fare ticket from Amsterdam Centraal to Berlin on that IC for 69€. A 1st class Savings Fare ticket from Amsterdam Centraal to Bad Bentheim on the same train, online from the Bahn, will cost you 49€. I don't think the difference of 20€ (about $27) warrants using a day of a rail pass. Just get the Savings Fare ticket straight through. You might be able to get the ticket to Bad Bentheim for less from Dutch Rail, so you should make that comparison, too. Just checked for you. Amsterdam to Bad Bentheim is also 49€, 1st class, advance purchase from Dutch Rail for that train.

By the way, if you don't advance purchase the ticket from Amsterdam to Bad Bentheim, the full fare pirce is 116,20€ (~$160).

You don't need to reserve seats when you buy the promo pass. You only have to have a reservation when you get on the train with the pass. You can get the reservation online when you are sure of the train you want or at the station just before boarding. However, if the reservations are all sold out, you can't get on that train (as you could with a regular pass or ticket), so advance purchase might be prudent.

It would be simplest, one-stop shopping to buy the advance ticket now for the whole Amsterdam-Berlin route and add a 4 euro seat reservation to that booking through DB.

Of course, extra days on the German pass cost as little as $12 per person in 2nd class or $16 in 1st (to jump from 3 to 4 days). You get 2 free seat reservations in the same booking with the pass special if you enter the promo code! If you plan to use a pass day, and want to use one of your free reservations for that route (because other departure plans may be less certain), you can buy the partial ticket at 49 euros, as Lee described.

I don't know the seat configuration of this IC train, but it could be like the one I took several years ago on that route, where 1st and 2nd class compartments seemed to be exactly the same.

Thank you both for your advice. I booked the 1st class train thru DB from AMS to Ber. Now, I plan to buy a 2nd class German Rail pass from RS for the rest of the trip that will be totally in Germany. I've traveled both classes over the years and know there is not a lot of difference on the trains; however, my honey had some medical issues this year and I want him to be able to rest for this 6 hour journey. As we get older, we make some cocessions and adjustments on the way we travel, but never stop traveling - and stopping along the way to smell the tulips;-).

Thank you both!
Pat