I am on the same journey. See the thread (small bookshelf speakers with nait 50/uq2)
Very hard to find a good one.
Less powerful amplification limits it a bit. In my case uq2.
Currently on loan: Epos n7. Very modern and transparent… good bass. But lacking the musical magic.
Apertura swing. Heard them with atom. Very nice. But they need power - best 80w.
As @Innocent_Bystander has mentioned slim floor standers don’t take up any more floor space than stand mounted speakers - and sometimes even less. If you are concerned about bass then this would be my choice. Otherwise I wouldn’t choose any speaker just based on how much bass it produces. There are much more important considerations which will vary depending on your own preferences and priorities. But if you really can’t do without bass then a slim floorstander is likely to get you nearer to your ideal than a stand mount.
Maybe not high enough up the league but I am very impressed with my Monitor Audio Bronze BR1 speakers. The BR2 have better bass extension but are rather taller (but still fit on a bookshelf).
On the basis that everybody recommends what they have, Graham LS5/9.
BUT answering the questions posed by @Chalshus would make things much easier. If by using the term bookshelf as an alternative to stand mount, keeping with Graham Audio, look at their website and see why, the size of standmounts varies so much. It possibly also gives and answer depending on your likes and budget. LS 3/5 together with the Sub 6.
But (again) your profile says Linn Kan, so are you looking for a similar sound balance with deeper bass?
Given bookshelf in its most literal sense, I assume that you’re looking for something small and without a rear firing port, maybe?
While they aren’t currently on a bookshelf, I’m very happy with my Sonus Fabers Sonetto I G2 and their down firing port. There is also the II if you want something a little larger.