I have a pair of the Kef Q350's that the OP mentions.
They are great speakers with nice, clear mids and highs and surprisingly good bass.
Part of that good bass comes from the rear port, which would not fare well in a bookshelf. You could try plugging the port and thwarting the design intent...but good luck with that.
The front ported Q100's are designed for wall mounting or shelves, but you'll sacrifice bass and midrange. Perhaps a pair of small subs that can reach fairly high can help, but they would surpass your budget.