Can the LSX II work as TV speakers?

Yes. It has HDMI ARC and an optical input, so it connects directly to most TVs and switches on with them. Dialogue comes through clearly, though it won't replace a soundbar with a dedicated subwoofer for big action scenes.

Do I need the app to use them?

You need the KEF Connect app for Wi-Fi setup, firmware updates and most EQ and input settings. Bluetooth and wired inputs work without it, but the app is part of normal use.

Can I add a subwoofer?

Yes. The LSX II has a subwoofer output, and KEF sells matching subs. Adding one fills in the low end that the small cabinets can't produce.

How does it compare with the original LSX?

The LSX II adds HDMI ARC, USB-C and better streaming support, and its app and platform are more current. Sound character is similar, so the extra inputs are the main reason to choose it.

Is it loud enough for a living room?

It plays comfortably in small to medium rooms at normal listening levels. In a large or open room, or for loud parties, it runs out of headroom and bass before bigger powered speakers would.

A pair of speakers, a power cable, an inter-speaker cable and a remote is not included; basic documentation is. Check the listing for the exact bundle, since some sellers add cables or stands.