15542 Hz (ETMY) and 15522 Hz (ITMX) are parametrically unstable at 40 W after ~2 hour lock even after ring heater power increase. Only 15521 Hz was successfully damped with ESD actuation.

After a few hours locked at 40 W, the 15.5 kHz mode began to grow for both ETMY and ITMX. I'd watched a few other modes rise and fall during the lock, so I didn't start trying to damp until peaks had grown by over an order of magnitude. I was able to quickly damp 15522 (ITMX) with 2 Hz BP, -60 deg filter, and saturating the drive. 

I was not able to damp 15542 (ETMY). Driving did slow its growth but I couldn't find a damping filter combination that actually damped. I started trying to damp ETMY ~1 min after starting ITMX, so there's a chance I could've caught it earlier, but as you can see below it was also just ringing up faster. I suggest next 40 W lock we turn the damping filter on from the beginning. 

Below I've tracked both frequencies in the OMC DCPDs, showing the PI growth and damping (or failing at damping). I attach a power spectrum showing progression as well. 

When it was clear I wasn't able to damp, I tried stepping the power down 40 --> 38 --> 36 W but the lock broke quickly during this (not directly from the PI); sorry Stefan + Lisa. I'm leaving the interferometer in the DOWN state.  I've edited the new PI wiki accordingly.