[Update]
With the June update I get finally a consistent result:
Output speakers set to 2.0
“include LFE in stereo downmix” enabled
LFE volume sets to 80%
Passthrough disabled
I’m able to add extra bass to the stereo amplifier output. I downloaded surround test files and even if not all of them reproduce the LFE channel (maybe the file itself) official test files (Dolby etc.) do. Same with the test movie I used.
I ran the April update at the beginning and had some issue (no LFE) but it seems solved with the June update.