Set amps ?

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.

Regards,
-- 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.

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).
Regards,

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.

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.