Focusrite advised you correctly. My Tascam US-16x08 has 'buffered' Line Outs (no pops). My PreSonus AB VSL44 popped my BX8 monitors, but good. And yes, the switches are on the back, and yes, it's a bit of a PITA to power cycle them in sequence, but I do it anyway, out of habit.
So, power cycle your monitors in sequence...or deal with the pops.
Those are, unfortunately, your only two choices. Sorry.
Edit: Sorry, there's a third choice. Use the Tascam. And a fourth. Buy a new Tascam US-NxN i/f.