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.

The following uses a DHT 300B SET Amp as a point of reference

SET pluses

Has warm and liquid tube sound - No harsh digital edge
Has incredible sense of presence
Very little listener fatigue
Midrange and high end tone are magical
You can change sound by tube swapping

SET Minuses

They generally cost more than other Amps
Very low power - requires higher efficiency speakers
Bottom End (Bass) is not very prevalent.
Tubes can be very expensive and need to be replaced
Tone is probably "colored" somewhat by the tube
Does not measure well (spec-out) compared to traditional Amps

My progression has gone from 20 years of owning "Sand Amps" to Push-pull Tube Amps (Much Better) to DHT SET Amps (Even Better). I currently have 4 SET Amps - A pair of 8 Watt 300B Monoblocks, a 300B Integrated, an Amp that can switch between 300B and 2.5W 2A3 tubes and finally an Amp that can switch between 2A3 and 1.5W 45 tubes.

To me, a DHT SET Amp (300B, 2A3, 45 etc..) is as close to "being in the music" as I have yet found. I attend at least one classical (symphony, chamber or recital) concert a month and SET amps best approximate the feeling of having the performers in the room with you.

However, there are few absolutes in music. The audio qualities I look for and favor may be the opposite with you. I strongly advise going down to your local high-end audio retailer and listening to a good SET amp matched with appropriate speakers. It just may lead you down a different path. There is probably a good reason why so many audiophiles have fallen in love with technology that is 70+ years old.