From what I understand , examining the speaker's impedance curve is the key - magnitude and phase angle .
If you look at the impedance curve for Troel's 3 way classic MK II , you'll see the magnitude of the impedance is 5 ohms and above ,
and the phase angle is well behaved . The amp almost sees a 5 ohm resistive load .
On paper , I figure the USSA - 3.2 would be a fabulous match to these speakers .
http://www.troelsgravesen.dk/SEAS-3-Way-Classic-mkII.htm
Recall, it takes current for a voice coil to build up a magnetic field .
So IMO its all about getting current from the power supply to the load ( speakers )
So a big enough power supply , and an amplifier with a low enough Zout is needed for this .
Usually , Zout is expressed as damping factor into an 8 ohm resistive load .
However , B&W's impedence curves are notorious for presenting a difficult load for an amp.
Its wild swings in the phase angle that give amplifiers grief .
I figure B&W 's where the minimum impedance 3.9 ohms , can be driven by amps with a DF >= 60 .
Like a H/K 3500 receiver can provide just enough current to drive these .
Where as B&W 's who's impedence curve drops down to 2.9 ohms - forget it . A beast of an amp is required .
I can't find the the impedance curve for the B&W 685 S2's . But the impedence drops down to 3.5 ohms .
So if a receiver can effectively drive these , a USSA 3.2 with a big power supply should be able to .
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Fab ,
To add to this , I'm getting the components together to build a USSA 3.2 to drive a 1990's pair of B&W 603 s2's .
I wanted to build the USSA 3.2 , because F5 type of amps are so well behaved , and straightforward to get up an running .
For the B&W 603 s2's , the Impedence Curve drops down to 3.9 ohms .
I know an H/K 3400 receiver - 25 watts and with a DF = 60 can deliver just enough current to drive these speakers .
Can not find the impedence for the 603 s2's , but here is the impedence curve for the 603 s3's .... which drop down to 4.0 ohms .
https://www.stereophile.com/content/bw-dm603-s3-loudspeaker-measurements
The jFET's I bought from DiyAudio Stores have an Idss = 6 to 8 mA . So with the lower Idss , hopefully this will push up the Damping Factor a bit .
Following this 2 part article from Erno Borley , somewhere in here , he says to set Id to 60% to 70% of Idss
in order to maximize linearity .
https://audioxpress.com/article/JFETs-The-New-Frontier-Part-1
.
To add to this , I'm getting the components together to build a USSA 3.2 to drive a 1990's pair of B&W 603 s2's .
I wanted to build the USSA 3.2 , because F5 type of amps are so well behaved , and straightforward to get up an running .
For the B&W 603 s2's , the Impedence Curve drops down to 3.9 ohms .
I know an H/K 3400 receiver - 25 watts and with a DF = 60 can deliver just enough current to drive these speakers .
Can not find the impedence for the 603 s2's , but here is the impedence curve for the 603 s3's .... which drop down to 4.0 ohms .
https://www.stereophile.com/content/bw-dm603-s3-loudspeaker-measurements
The jFET's I bought from DiyAudio Stores have an Idss = 6 to 8 mA . So with the lower Idss , hopefully this will push up the Damping Factor a bit .
Following this 2 part article from Erno Borley , somewhere in here , he says to set Id to 60% to 70% of Idss
in order to maximize linearity .
https://audioxpress.com/article/JFETs-The-New-Frontier-Part-1
.