Went ahead and decoupled the YTM and shifted it away so I could connect a cable to the low pass filter.
The output power of the power amp was at most +15dBm and dropped around 3 GHz to +5 dBm, way less than the constant input power between +11 and +13 dBm.
As the power amp has a rather low serial number (0365) I'm assuming it is of the "sensitive" type so I ordered a replacement.
I should have measured the amp output power without the isolator and will do that once the replacement amp arrives.
