As with everything else, it depends. If you can find a quality two-channel amp that makes the power you need at the impedance you need then it is good. If a mono amp can do the same thing it is good. Most newer mono amps are class D and have a rather limited frequency range which eliminates them from use as a mid-high amp. They are also generally more powerful and much more efficient than a simularly priced two-channel amp and this suits them to sub duty. If you are looking for really high power, mono Class D is pretty much the only way to go. For a modestly powered system, a two-channel amp can drive subs just as well as mono.