Yeah, I definitley plan to have a subwoofer. Would you say the X2700H is adequate enough for the LS50's if not playing them at high volume? Or any other more budget receivers that you can recommend may work with them?
Yeah, I think the biggest concern is not being able to source a center when the time comes that I decide to get one.
The speakers have a relatively low sensitivity and can get low in terms of impedance at times which I think has made some people suggest that you need a powerful amp to drive these. If you were listening to music mainly at higher volumes in a large room with no subwoofer then, yes, I'd tell you to upgrade to get the best out of them.
You're not doing any of that. And you're getting a subwoofer to help carry the load. Even with another Centre it should be fine
But the x3700 does have advantages other than power - only you can judge if they're worth it.
Read our detailed comparison of the Denon AVR-X3700H vs Denon AVR-X2700H to find out their common strengths and weaknesses, advantages and disadvantages, and decide which one to choose.
You could just go and listen - the speakers and AVR are very popular models so you may be able to do that quite easily - or even go for the X2700H with an understanding of swapping it out for an upgrade if it's not quite right.