Are low-power amplifiers acceptable in hi-fi systems?

It depends on the system, sometimes yes, usually no.

The sound pressure levels achievable with a hi-fi system are down to two parameters: amplifier power output and speaker sensitivity. Low-power amplifiers can create realistic sound pressures and dynamics, but only when matched with sensitive speakers.

Do not expect a 30W per channel amplifier to offer high fidelity dynamics and sound pressures with normal sensitivity speakers though. It’s not physically possible, no matter what anyone might tell you. Hi-fi listening at low levels may be possible here, but scale and gravitas will be missing.

This FAQ covers the concept in more detail.