Bookshelf speakers-double up,kef ls 50

Well, they may play louder but, otherwise, it would be a mistake. Using two per channel will corrupt the excellent imaging of these speakers as the stacked pair will be very sensitive to vertical angle and vertical head position.

If you want louder and/or deeper, add a decent subwoofer.

Place them back to back and you'll get a bipole dispersion pattern. If you place them correctly you can achieve a very deep soundstage. A point source speaker like the KEF would be perfect for this type of experimentation.