Is this analog method the same as if I only use one output on the H90 or are there sound differences, possibly due to dedicated monophonic algorithms?
No, this is not the same. External mono summing is good enough when you’re in a bind and need to work with a mono summed signal. For example, you’ve wired up your board in stereo, but now face a situation where only one amp is present. In general, for optimal performance, you should try to come out of the H90 with one cable from Out 1 only in insert mode or Out 1 and/or Out 3 when using Dual routing mode.
Check out this video for an in-depth explanation: https://youtu.be/a6XH5zEeR4o?feature=shared
