Brickwalling and clipping are two different things...you can brickwall without going over 0db.
Brickwalling is what happens when you're running your levels too low into your deck (mic in) and making the deck's preamp overload; this typically happens below (I think) 4.5 on the knob. That's why you always want to go LINE IN and bypass the internal preamp (and still feed your deck as hot a signal as possible).
And as bean said, *audible* clipping definitely doesn't always occur on an over; on a transient over like a cymbal crash or audience clapping, you won't notice it because it's such a quick spike. When you're constantly riding into the "OVER" lights, that's when it becomes audible.
--Dave