The CBM-170SEs are very good, but you DEFINITELY need a sub with them. The 340s are still somewhat lighter on bass than a typical bookshelf of that size, but you can get away with them without a sub in a smaller room IMO. With a decent tower, I tend to listen to music without a sub even giving up that extension (somewhat for simplicity), however my main rig has been bookshelf/sub for many years.
Stumbled on this thread while looking for reviews of the new Elacs.
Anyway, I have the CBM-170SEs and the characterization that they have "no bass" and "you definitely need a sub" is IMO exaggerated.
They are specified to have typical in-room response down to 53Hz (-3 dB). I just used a tone generator and, at my normal TV-watching volume (not especially loud), they can produce a nice meaty tone at 40Hz and still go somewhat lower than that.
It must be remembered that many people on this forum have extremely high standards, and if their sound system doesn't have the bass response of a THX movie theater or a dance club, they consider it unacceptable.
Meanwhile, in the rest of the world, most people still watch TV with the built-in speakers, which usually struggle to produce any sound at all below 70Hz. Compared to that, the bass from a CBM-170 is spectacular, rather than nonexistent. Most "sound pedestals" (like ZVOXs) only go down to ~50Hz and many of those are considered quite good. And there are über-expensive sound bars from vendors like SONOS and B&W and Definitive Audio that don't produce
nearly as much bass as CBM-170s but reviewers rave about how great the bass is from these small bars and how you don't need to use a subwoofer with them. And I should also mention that the tiny subwoofers that are bundled with inexpensive sound bars and HTiaB systems often aren't specified below 50Hz either.
So yeah, you need a subwoofer if you want your living room to sound like a movie theater. The CBM-170s will not get you there. But that isn't the same has having "no bass" or absolutely requiring a subwoofer.