Well, performance mode should work as soon as the master volume is registering sound through the speaker.  It's just about keeping the noise from that speaker under control.  You could run an external speaker inside an airtight sealed box with a mic inside or better still a box within a box which is isolated from the outer box with vibration absorbent blocks (rubber/neoprene or similar) then place the box on a soft absobent surface, all of which should make the speaker much quieter.  I'm talking about an isolation cabinet here - and If you can make it all airtight the better it will be.  Glue and screw all the sides, use rockwool if needed, but an air gap of a couple of inches between the inner box and outer box except for the points where the inner box sits on the absorbent rubber/foam blocks should guarantee a healthy reduction in volume.  If you use your amp a lot at home for recording this might be an idea worth pursuing.  I just received next month's Sound on Sound today in the post and someone was asking something similar in the Q&A section, so was I reminded of the isolation box idea.  Randall make such a box professionally as do others, but you need to be sure to get something that is airtight as they are prone to sound leakage if there are any air gaps.  See here: http://www.randallamplifiers.com/Cabinets/Isolation-Series-Cabs/Isolation.html

Nick