You get a lot in one rack space
I have been using an ART SLA-1 power amplifier in my rack based live performance sound system for about three years, because size and weight are critical factors in all my equipment choices and the power output is more than sufficient for any venue where I sing. When I determined that I really needed to have a separate channel for my stage monitors, I chose the SLA-4 based on my extreme satisfaction with the SLA-1. So far, I have only used it for two performances, but the operation and performance are identical the SLA-1, but with the addition of two more channels. Hey, i could add a subwoofer! The toroidal transformers produce great power with significant weight savings from conventional transformers. The cooling fan is slightly louder than the fan on the SLA-1, but then it is cooling four channels instead of two. It is not annoyingly loud, or even really noticeable except by the operator, and then only when the room is otherwise silent. I highly recommend this amplifier, especially for a traveling sound system when size and weight are considerations.