the stiffer ride might have finished off an engine mount (2 new replies posted while typing, doh!), possibly maybe - also if they know their stuff they probably already cut down the bump stops but it's worth asking them as you could be bottoming out on them causing a knock. The other thing being used springs you could be bloody unlucky and had one of them snap already
i take it you've only just noticed as you never mentioned it on saturday, could've had a look for you