What are the benefits of a SET amp?

Like anything there are trade offs. But in general the sound is typically very lifelike...lots of air, weight, almost a wetness if that's a word to the sound. Most SET's are low powered amps and take careful system matching...

In my opinion, there are no benefits, unless you like distortion and bloat. Not to mention that there are very few really great speakers that are affordable and will work well with SET amplification.

There are lots of high efficiency / high impedance designs available, many of which are very affordable. The problem is that most of them sound awful. SET systems are fun toys to play with, but I wouldn't recommend one for critical listening. It's just not reference quality stuff.

It depends on the design of the amp and the system, but other than what Jfrech describes above there can be a 3D quality that I have yet to hear from non-SET amps (this includes good push/pull tube designs as well as SS amps running in Class "A". If you are 45+ years old you may have heard these setups when you were a child as these amps were common then (single ended if not single ended triode) in many radios and consoles built prior to around 1960. Efficient and/or easy to drive speakers (the specs don't always tell the real world truth) are a must if you listen to demanding music or have a large listening room (even @ moderate volume).

SET abbreviation means Sentimental Every Time.