Solo boost position in the signal path?

Does the solo boost happen before or after the effects loop send and return?

I have a non-plus Express 5:50 (no solo feature) and am considering adding a 5:50+ or a Five: 35. With my 5:50 I've gotten great results from having all pedals in the effects loop, including a loop station. Since any boost would have to come before the loop station and would cause problems being after the loop station (and effects loop in this case), I'm wondering if the solo feature would be in front of the send/return in the path and thus usable for my configuration.

Thanks in advance for any replies.

The Mark V has the solo boost as part of the master volume control. Runs in series with the master and is bypassed by a relay when turned off. This is following FX loop and just before the phase inverter circuit. This may be similar with other Mesa amps with the solo feature but unsure if this is the case. Not much help regarding the smaller versions of the V.