https://skyfiaudio.com/products/electronics-service-labs-esl-bridging-amplifier-device
If you are DIY handy, use a transformer is an easy way to get the job done.
Stereo to mono bridge/phase inverter for AVA ST70 amplifier
I have 2 AVA ST70 amplifiers that I want to use as mono. AVA did offer a stereo to mono bridge/phase inverter in the past. AVA now says that they don't supply this and are not sure if anyone is making a phase inverter suitable for hi-fi. I would appreciate any help on this matter.