After a very lengthy Google search I've found that I perhaps made a
mistake in purchasing floorstanding speakers and should have went with
bookshelf. My main problem is that the speakers are about 40cm too
low.

I gather it's agreed I'd have been better with the bookshelves, but now
that I'm stuck with floorstanders, would there be anything about
raising their height with stands that would actually take away from the
sound quality rather than just loose the benefits floorstanders have?

I get the impression if I had a 40cm stack of paving slabs under each
it would have the desired effect. If so, is there an alternative that
would be better?

Preferably I'd put them on some coffee tables but worry they would be
unstable and would badly affect the sound.

Thanks in advance for any info.