Uber FX : Very poor sound

I bought this for latin dance events for smaller groups of friends now that Covid19 has ruined all big social events. I've not been able to put in practice with other people as the sound was terrible! I connected it at home with Bluetooth playing through Spotify and the sound was very weak and dissapointing. It was like some middle frequencies were missing or something (not an audiophile so I'm guessing about the freq spectrum).

My first thought was to compensate for it using the EQ only to find out that the EQ is only for the linein/instrument jacks and not for Bluetooth.

I made a last attempt to jack in my phone in one of the linein jacks (setting it to linein mode not instrument). The sound was even worse! Playing an AC/DC song I could hardly hear the vocals! Now being able to use the jack's EQ though I made some quick adjustments but it was futile. Nothing worked to get a decent sound out from it.

Very dissapointed but also surprised it was this bad.

(Setting Battery Life & Quality as 3/5 stars as they are mandatory but I've not been able to test these.)