So, it wasn’t the speakers? Imagine that! ;)
Emiliop, congratulations on your discovery. Yes, you are learning that within reason, i.e. operation limits, every variable is to be tried. Gain switches can be very effective in contouring the sound. I presume that if you have triode mode, you also have an alterntive mode. Also try Ultralinear vs. Triode modes. Be sure you check the Owner’s Manual for any possible combo that is not recommended, i.e. certain speakers that are less efficient in combination with settings of the amp.
I’m sure you are having fun now! It’s terrific fun to explore the different setups and to hear the outcome! Remember what I said, the performance spectrum is enormous! You still are playing in a very small slice of it. Actually, any of the types of condifgurations done with one set of gear is de facto playing in a very small portion of the performance spectrum. There are dozens upon dozens of steps in sound quality improvement available to the audiophile.
Oh, BTW, get in the habit of reading Owner’s Manuals before you use products.