Set amps ?

How do these 2-6 watt amps work ? Can they drive proac response 2.5 speakers . What is so great about them?. What makes them tick ?
That Speaker is considered an easy load at 8 ohms with a dip to around 6.5 ohms in the lower midrange region. The sensitivity is only 83 db as measured by stereophile.SET amplifiers range in quality and performance as do all amplifier types.Given the very low sensitivity of 83db, the room size and your listening levels will matter a lot with the lower power range amp(limited volume and headroom) you`re considering.

The better SETs have robust power supplies and premium quality transformers(along with better parts,wiring etc.) as opposed to lower quality SET amps.
You `ll likely have much better sucess with a higher power SET,certainly more flexibility,The better built SETs can sound wonderful with the appropriate speaker match.
I agree with Charles, 6 to 8 watts won't do it. A 20 to 25 watt SET might be enough, as benign impedence, can be more important than sensitivity, with tubes. If you go that route, it would be very important to try the amp with the Proacs and in your room, before buying.

I have had a couple of 20watt SETS and can recommend the Ayon Spark, which I use at the moment. Almarro do a nice SET amp, at a lower price, both are excellent. The Viva Solista I used before, is very expensive now and I think the Ayon is as good.
6 watts will work fine a lower levels in a small room, but if you want to bring these speakers to life consider a hundred watts or more.
I don't believe I would never partner any Proacs with SETs. The brand has always loved power. I agree with Tmsorosk - 100w plus.

What is so great about them is that they are usually a very simple design employing Class A operation which can pass a less bastardized audio signal to the speakers.

What's not so great about them is that they are low power (you asked about 2 - 6 watts) so you need to mate them with the right speakers. From the responses above I don't believe the Proacs are the right speakers.
A simple design rarely equates to better sound. The bastardization occurs when designers don't implement circuits correctly.
Agree with Sebrof,
Straight forward and simple circuits that require fewer parts sound natural and very realistic,just more like what I hear at live venues. I respect other opinions that disagree,I`m going strictly on my actual listening experiences. Many complex, higher power amplifiers sound artificial and hifi to me(they are`nt convincing). We all hear differently I realize and there`s something available for all of us.Good SET gets it right for me but will not do it for others.
I would say the reason why well implemented SET amplifiers sound so good is due to a simple circuit that retains much signal purity. This results in a very natural sound and purity of music.

In the current 6 there is a review of the new Avantgarde XA preamp. The chief engineer explains the mandate was to make the circuit as simple as he possibly could.
It has only one gain stage,only one transistor per side,no coupling capacitor or DC servo unit(it is DC coupled instead).He made the signal path wiring ultra short.

What is interesting is the reviewer points out that the Soulution 720 preamp follows and is built on this very similar 'simple' concept. It`s a good read if you have the time.

Nelson Pass has ventured down the path for quite some time with his First Watt amplifiers.They have become simpler and ever better sounding over the years. His latest S.I.T. anplifiers are the most simple yet and are said to be the best sounding of all the First Watt designs.

There is much merit in the keep it simple concept and SET represent the epitomy of that idea.The principle is lower the number of parts used while increasing the quality of them.That works for me.
In addition to the power consideration, I would be cautious in pairing a SET amp with a speaker whose impedance varies widely as a function of frequency. See Figure 1 of Stereophile's measurements of your ProAc Response 2.5.

Basically, the high output impedance that SET amps typically have (corresponding to very low damping factors, damping factor and output impedance being inversely proportional) will result in increased emphasis of frequencies at which the speaker's impedance is high (in this case, particularly in the vicinity of 1 kHz), and decreased emphasis of frequencies at which the speaker's impedance is low (in this case, particularly in the 100 to 200 Hz region).

The fact that the speaker's impedance goes no lower than about 6.5 ohms at any frequency will mitigate that effect to some extent, but I would expect it to still be audibly significant with most SETs.

The excellent results Stereophile reported when using the Cary 805 SET amp with your speakers should not be viewed as indicative of the tonal balance that other SET amps would provide, because the 805's output impedance is much lower than the output impedance of many and probably most other SETs. See Table 1 here.

-- Al
Art Audio Jota High Current (HC) has 24 watts per channel with KR 300BXLS tube, and will drive your speakers with no problem. SET amp sound is magical.
I love single-ended amplifiers but I've run into matching problems with lower efficient speakers. Even a homemade 15W SE (ultralinear in this case) amp with negative feedback couldn't drive a pair of current hungry Magnepan speakers to any reasonable level. But they did sound quite magical at low volume.

But the same amp on 91dB KEF speakers makes plenty of volume - in my friend's smaller living room.

So if you like head-banging levels or big dynamic music, then SE amps may not be the way to go, unless you run some "big powered" 805/845/211 amplifiers or get more efficient speakers.
As usual salient comments from Almarg. I would agree this Proac speaker is`nt a likely choice for 'lower' power SET amplifiers.However there are some SET models out there that will mate with this speaker superbly. Too bad that it`s usally difficult to get these various amps into your system and hear for your self. As Jburidan says the sound can be magical(with the right speaker).
The Response 2.5 works great with our M-60s, but as I recall some customers could run them (60 watts) out of power, although they were in a large room. 6-8 watts just won't be enough.
Most here agree, if you decide to try a SET with your speakers you`ll need the higher power versions(with good transformers and beefy power supplies).
what was magical, albeit not tubes, was Rowland with 2.5s. A simple Concentra was sublime on them

of course, ARC is what they are designed with.
Thanks for the responses. Arc is what i aim for some day when the budget allows . Until then lower cost amps and preamps are in the plans.