Matrix went with a signal light which flickers at -9db and remains on at -6db. That equates to 50W when flicking ans 100W when stable into a 4 Ohm load, 30W and 60W into an 8 Ohm load, then 15w and 30w into a 16 Ohm load. While with a 16 Ohm load thats not much use - the Max OP is 130w anyway so should be safe for most 16 Ohm cabs. With 4 and 8 Ohm loads it was deemed more useable as it gives a guide of OP power at or around the powers cabs are more suited for while lesving enough headroom.
Thelight is very useable. Set the AFX up using the gain slider int he global Out menu while both AFX and Matrix are up full so the light on the Matrix flickers when playing your loudest patch hard (with any vol boosts engaged). At that point you know the power the amp is pushing into the speakers. Then raise the gain slider by however many DBs you need to provide the most power your speakers can handle. That way when both hardware vol pots are up full - your at the max volume your cabs will safely handle. from there you control your actual volume from the Matrix gain knob (on the Gen 1 units I used the AFX knob but on the 2 you get that zipper noise doing that so I prefer using the Matrix knob).
for example, if you have a 2x12 cab rated at 8 Ohms loaded with a pair of V30s, you max RMS power handling is 120w. Set the Gain slider of the OP connected to the matrix at its lowest (-12db) set the Matrix knob to its lowest and the AFX knob to its highest. Start playing and raise the Matrix knob to full (you can actually do this with no speaker attached with SS amps - so no danger of damging your cab or blowing your ear drums during the procedure). If the light starts to flicker while raising the Matrix gain to full your patches are too hot - back off the patch gain. Once the Matrix is up full - go back to the global output menu and start raising the gain slider until the Matrix light flickers while playing. At this point your puttong 30w into your cab - if the light goes to permanantly lit quickly (from off) thats fine - it means you putting 60w into the cab - but your better off with the light flicking. So - now you know your at 30w. the cab handles 120w. Each 3db raise doubles the power - so raising the gain slider in the global out by a further 6 db will then push the full 120w into your cab (3 db up from 30w = 60w, then a further 3db up from 60w = 120w). At this point your set. Provided you use the patch as your "comparison" patch and dont make one thats louder - the max you can push with bopth Matrix and AFX front pannel knobs up full is 120w - which matches your cabs handling. If you get the odd spike more than that (possible if you play harder with the adrenaline of a gig) its not an issue - as guitar speakers can handle spikes above their rated power (the manufacturers know for example a 50w tube amp will push up to 80-90w when the power section is pushed hard - so take this into account).
