They did make an announcement recently saying it invalidated the guarantee. They may not have updated the manual yet.
I've never connected speakers to mine and poignant never will.
If I was going to use an amp with the TT2, it would be something with tubes and color. That would make a lot more sense to me given my experience with it. I would then be able to use the TT2 for that clean solid state sound and the tube amp for something different.
It doesn't make sense to add something like a monoprice THX amp in front of the TT2. Why would you add power to something already more than powerful enough and then just end up attenuating the source signal? Surely you are hearing less of what the TT2 is putting out in that case. If you enjoy that, more power to you, but it's not wrong to advise against it.
If you're going to use an amp with the TT2 and use the amp's volume control, you might be better off getting a MScaler+Qutest, that would make more sense to me.
I'm planning on Woo Audio 3ES