The X770 (RS520) is the previous generation of the X790 (RS540) and was released in 2017, The X970 (RS620) was the top model in 2017 and was replaced by the X990 (RS640) which has also been discontinued. All of those are excellent projectors with the X970/990 having the best contrast ration JVC has ever had. When the native 4K models (NX5/RS1000, NX7/RS200, NX9/RS3000) came out in late 2018 the X990 was discontinued but JVC kept the X790 in the lineup as a less expensive model.
The X770 (RS520) and X970 (RS620) are both native 1080p panels that accept a 4K signal and use JVC's e-Shift technology to provide "faux-K". I've provided more info on e-Shift below. The NX5 (RS1000) is the cheapest JVC Projector with a native 4K panels (and second cheapest overall - only to Sony's $5K 295). For whatever reason, the JVC 4K models do not have as high of contrast as the earlier models with the 1080p panels, but they still have superior contrast to virtually any projector on the market (The $$$$$ 330lb Christie Ellipse excluded).
Any of the JVC's would be great in a home theater, and all have amazing contrast compared to the competition. It won't have OLED blacks, but in a dark room, it will be close enough, and you'll feel like you're really watching a movie, not just a TV.
7.4.4 Theater Room: JVC-RS500, Silver Ticket AT 2.35:1 142”, Onkyo RZ830, Anthem PVA-7, Panasonic UB420, Apple TV 4K, JBL Studio 530’s, Dual Driver VBSS
3.1 Living Room: Samsung 64” F8500 Plasma, Anthem MRX 300, Dynaudio Audience 52’s, Dynaudio Audience 122C, NHT SubOne, Roku Express (2019)