B&Ws are traditionally not tube-friendly speakers. IME, a fair bit of performance of the tube equipment in use in this system already is left on the table on account of the speakers being harder to drive.
When shopping for speakers, look at what the manufacturer uses. You don't have a lot of power here, so you want a speaker that doesn't need much. Eight ohms would be the least impedance (particularly in the bass region- stay away from speakers that have dual woofers unless you know that the combo of woofers is 8 ohms or more) and the efficiency should be at least 92 db in most rooms (if a small room less efficiency might work).
When shopping for speakers, look at what the manufacturer uses. You don't have a lot of power here, so you want a speaker that doesn't need much. Eight ohms would be the least impedance (particularly in the bass region- stay away from speakers that have dual woofers unless you know that the combo of woofers is 8 ohms or more) and the efficiency should be at least 92 db in most rooms (if a small room less efficiency might work).