If you have your mind set on X1 and B3 Pro I, you don't need an amp to pair it up. As I mentioned in my X1 review, I had B3 Pro I pumping connected directly to HO of X1 with 50% volume setting. But to make it shine, just like if you want to make any IEM or full size can shine from X1 - you will need to switch X1's HO to LO and connect an external amp. Either way, investment in external amp is a good idea. If you are on a budget, E11k is a good solution or you can go even cheaper with Topping NX1 (better battery life, though don't think it has bass boost like E11k).
My suggestion, unless you are looking at a discounted X1 + E11k bundle, I would just get X1 and see how you like it with B3 Pro I. If you are happy, there is no need for an amp. But in a future you can always upgrade this modular system by adding external amp which going to make X1 shine for sure.