SET abbreviation means Sentimental Every Time.
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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).
The following uses a DHT 300B SET Amp as a point of reference
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
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.
My, how the SS crowd is getting so aggressive these days. In an attempt to remain impartial(ha!), I will state that there is a group of purist type audiophiles who revere this type of amp for it's purity of midrange character, and are willing to accept some shortcomings in the other areas in order to get this golden midrange. Since the ear is most sensitive to the midrange frequencies, and most of the musical melody fundamentals are in or near the midrange, this makes some sense. In some of these designs, due primarily to the behavior of most output transformers, the high end gets somewhat rolled off and possibly some tubby bass can occur. This is not the case with all SET designs however, and the best ones are very good at controlling the inherent difficulties of the design. They are mostly lower power types, but not necessarily so. The typical SET system will also have high efficiency speakers, maybe single-driver types. Contrary to the above ravings of other posters, the best SET amps have the best midrange quality of any amp type. Few serious people will argue against that point. Most detractors argue that the highs and lows are not handled as well as the mids, so they disqualify the amps as flawed in design. But, since no amp is perfect, if you focus strictly on the flaws, you wouldn't be happy with any amp. Many SET owners, when challenged with this argument by SS proponents, will state that SS amps provide a wide frequency spectrum of flawed grainy sound that doesnt even give good lifelike midrange, so what good are they? The truth lies in the execution of the design of each particular amplifier, and in the tastes and sensitivities of the buyer/listener. As for myself, I have owned and enjoyed SS amps for 25 years, but now have an OTL tube amp and would never go back to SS again. IMO, there is no comparison in sonic quality between them. It won't play as loud as a big SS amp, but for the SPL it will provide, there is no SS amp made that can match it for musical "sweetness". And that is just a simple, natural born fact. The only thing that SS really has in its favor is big, honking power. For delicacy, detail, and naturalness of sound, it is really no contest.IMHO :-)
If you value musicality and emotion SET is the only way to go. They only work well with high efficiency speakers such as most of the Meadowlark line to name one example. These type of speakers also seem to have the the same qualities as SET giving more emotion and musicality than those with low efficiency and more complicated cross overs. If you are more interested in high fi effects stick with SS or push-pull. If you really value music SET with proper speakers cannot be beat.
The major benefit of a SET amp is its ability to show what the regular tube amp and ss amp are missing in music reproduction. Of the 3 types of amps, even though the SET amp has the worst performance in test measurement but it sounds closest to the real thing(with the right speakers)! So another benefit of SET amp is its ability to show that the test measurement for testing amp are useless in determine the true performance of the amp. Of the 3 types of amps, the ss amp nearly has perfect test measurement but it sounds the worst.
My amp progression was SS reciever followed by three tube amps ending with the Moth s45, which is a custom SET amp that uses either 2a3 or 45s. The three tube amps were used with a speaker - Silverline Sonatina - which are not typical SET fare (93db). After three months with the Moth I've found that the Moth+Silverline marriage is not only compatible but improves on the presentation of the tricked-out 32wpc pp VAC. YMMV and all that jazz but what the Moth does is closer to my replay preference than any other amp or even component I've heard.
The Moth SET delivers on the SET virtues reported by end-users; the sense of naturalness, micro-dynamics, detail and listener ease is simply on another level. Where you STOP observing frequency behavior and focus on the music. With my less than ideal speaker pairing headroom, bass slam could be problematic but not for me as I do not listen much in the 90db range. With the above SET characteristics I'm less inclined to crank it up to seek what some look for with higer volume; micro-dynamics. Instead my head and feet bounce along to the propulsive musical pulse, this is especially true with the 45 tube (New Order). Though the 2a3 bass carries more weight and authority (Led Zepplin).
The fact is the SET amp causes you to rethink your priorities in replay, least it did with me. Actually, it affirmed the direction I was seeking. I will consider more "SET-friendly" speakers (horns / single-drivers) at some point but till then I'm enjoying a greater level of replay performance and listener enjoyment that before. Not for everyone, but for me everything else I've heard reminds me more of electronic artifacts than music-making itself.