There's full range and there's full range. This is a term that is often misused by speaker manufacturers. The Quad 21s are not even remotely close to being full range enough to do without a subwoofer.
I have only heard one pair of speakers that I thought were truly full range, in fact they were being used in the mastering suite at Metropolis Studios as full range speakers as part of a 5 channel setup without a subwoofer.
Bass is tricky and subs are no exception. A simple 50p component in the crossover will limit the bass drivers frequency response whatever it's size and the speakers size. This is used to get that combination of visual wow in a large speaker without the bass driver either working outside it's comfort zone or trying to produce bass its enclosure was not built well enough to deal with.
This is why you cannot determine a speakers bass extension and quality from the size of its driver, its size, or, I'm afraid the technical blurb.
This is actually a good thing, effectively limiting a speakers bass extension is a tried and tested way to avoid bass colouration and give a tight and fast bass response, though of course not low enough for movie effects.
So you need a sub, so make sure you get a high quality unit with a well constructed cabinet that actually can go low. Unfortunately with subs the marketing emphasis is always on how low they go, and some manufactures will play on this to the detriment of quality tight and fast bass.
I'm gonna disagree with the rest of the lads: you don't NEED a sub, but it does help fill in the low-end with (some) films. Those Quads make enough bass on their own for films to be very enjoyable & will probably sound better without a sub with music. You need to decide which is more important; good music reproduction or really deep bass when a helicopter crashes?
I'd suggest the only hiccup with your Quad 5.0 choice is the centre speaker - the tweeter is a newer version then the one on the older 21L, so there may be a tonal mismatch across the front 3.
Crusty's not heard the Dynaudio Evidence range.......
A sub isn't a total necessity. Yes, a full range budget hi-fi speaker will produce enough bass for you to enjoy music and movies in and average size room. But once you hear a good sub and the difference it can make to the impact and scale of a film, it's pretty hard to turn back. A decent sub will produce frequencies lower than the average hi-fi speaker can only dream about.
I'd spend the money on the 5.0 setup first and get that side of things sounding good, then you can worry about the sub later.