You can easily test my second suggestion without buying a remote volume control. Using the IP Control app with the 218 just set the analogue output to fixed and feed the B&Os (hopefully not deafening!). Then vary the SpeakerLink output in the usual way down to zero. If the B&Os stay on then all you need is a remote volume control (£13.67 here in UK, from China on eBay) interposed between the 218 analogue out and the B&Os and you will have what you are seeking.

Cinema/Music: Trinnov Altitude32 8-16,218,Cloud 4 channel, Marantz UD5007,DSP5200SE(2+C),DSP3300(4),DSW(2),271,MShort Ceilings,AppleTV 4K
TV: Marantz SR7013 Wharfedale Diamond 9x4+Celestion(C)
Music: Roon,i7NUC8gen,QNAP TS253A, Sony Scala, MS200, Garrard DDQ650,Wharfedale Diamond 12.3
Kitchen: AD88 (bluetooth via i80).