Although the monitoring system doesn't form part of the direct recording chain, it certainly does influence the end product because any deficiencies in the monitors will colour your judgement and affect the way you record and mix.
Monitoring consists of two components - power amplifier and loudspeakers.
Power amplifiers are incredibly good these days, and although it is possible to hear slight differences between different models, these differences really are very small indeed and hardly worth considering. Basically the two requirements of a power amplifier are that it is adequately powerful for the job and it is reliable. A third nicety is that for studio use it shouldn't really have a fan. For nearfield monitoring a 100 watts per channel amplifier will offer adequate headroom. When judging power amplifiers, bear in mind that an amplifier that is quoted as 100 watts into 4 ohms may only give 50 watts into 8 ohms.