The idea of adding a separate power amp into your guitar rig is to amp up the signal. Generally, the more power you have means the louder you can go. However you want to make sure that you can turn up your amp loud enough to sound good but not so much that you distort the sound into unwanted effect or blow your speakers.
The pure clean sound you’re looking for can depend on your environment, and as with other components, there are many options for power amps to go with every different setup.
The QSC RMX5050a 2000 Watt Two Channel Power Amplifier has remarkable power output, sound quality and reliability.
The Electro-Harmonix 44 Magnum 44W Guitar Power Amplifier is an inexpensive pocket-size replacement for much larger and heavier amplifiers.
The ISP Technologies Stealth Ultra-Lite 180-Watt Pedalboard Power Amp is a high-quality power amplifier with nice controls and a compact size.
The ART SLA1 100W Power Amplifier has clean quiet power and consistent great sound.