That is an interrupt loop, Nik shows a few different loops for different models, but they are all interrupt. It interrupts the circuit right before the phase inverter to allow you to insert effects.
Buffered loops are definitely more friendly for both pedals and rack gear. Otherwise it's all a matter of either the gear works at the line-level signal the amp is putting out.
Ultimately....you can install the metro kit and have a SS buffer; se the Klein-uator for an outboard buffer; Weber sells a parts kit with a suggested schematic for a tube-buffer loop; Fuchs has some options; and there are various other kits out there. There is easily enough room in a 1987x chassis to install something permanent.
On the cheap, I've had success merely using one of these as by FX loop buffer. They aren't made anymore, but usually go for dirt cheap. I just nulled out the gate controls.
The Boss LS-2 is the Boss equivalent that should offer the same functionality and allow you a Return level control.