You do not "need" and amp. IF you happen to be listening to your current setup at such a volume that the amps in the Denon are Clipping (distorting), then an outboard amp may be of assistance, or a bigger AVR. Another being if you are running your speakers full range, then a powered sub and crossing the mains at 80hz would lessen their draw on the Denon.
First let me answer your question with some questions:
Are you dissatisfied in some way with your current system and/or sound?
Does it play loud enough for you
What model of PSB speakers do you have?
Is this a 2ch or multichannel system?
People usually add external amplifiers to get more power and play louder, cleaner, and safer. If you like it loud and your amplifier(s) can't supply enough power, it will distort the sound. Distortion from playing an underpowered amp too loudly is called clipping, and can damage your speakers. It's better to have too much power so some is left in reserve (called headroom) for the loudest passages. AVR's usually don't have enough power to drive a system to undistorted reference levels.
I found conflicting info about the power rating of the Denon X2000. Beware of incomplete specs. Many AVR's and amplifiers try to inflate their ratings with non-standard reporting practices. A standard spec should read XX watts per channel into Y ohms all channels driven with no more than ZZ %THD from a to b Hz. Anything less, and the numbers are probably artificially inflated to make a product look better than its competition to the uninitiated!