[matt_j]

I play psy-trance at a range of tempos, from dub stuff at 70-80bpm, through prog stuff around 110-130bpm, to more high energy stuff up at 140-150bpm.

The main reason for sticking to a tempo is because psy-trance, and (good) trance music in general is about the constant pulse. The goal is to put the listener in 'the zone' (a trance even :P), and then give them a bunch of ear candy in the mids and highs to keep their happy little brains occupied.

So for any given set DJs generally pick a tempo and stick to it and deliver a driving, consistent pulse, and everything else is just colouring in on top.