Take a look at the Proac Studio 140 impedance specs: 8 ohms nominal, 4 ohms minimum. Not an ideal impedance curve for SET (or many tube amps).
For tubes, you want a speaker with high sensitivity and a flat impedance curve (with a minimum impedance no lower than 6 ohms).
For tubes, you want a speaker with high sensitivity and a flat impedance curve (with a minimum impedance no lower than 6 ohms).