While I bought the KEF Reference 3s, I seriously considered the 1s. First off - as an owner of the brand/line, I am, of course, biased. That out of the way, driven properly, the 1s are very good. Whether or not you will need/want a sub will depend on 1) music preferences, 2) volume preferences, 3) related equipment, and 4) room volume.
The 1s realistically go satisfyingly down to 40 - actually a bit below. Do the 3s and 5s go lower? Yes. Louder too. But, the 1 offers a surprising amount of bass. Unless you’re trying to recreate the full wait of an orchestra at volume or something like that.
If you do go with the speaker and desire to add a sub, make sure it’s of comparable quality. You could also live with the speaker for a while to see if it meets your needs and add a sub later if desired.
Good luck,