While soloing these on THF (hard, but doable), I found several, most actually, checked DC and gave roughly 95~ XP. So the level range has to be slightly lower, perhaps 49-51.
Also, these particular enemies are indeed Truesight, they aggroed me through Tonko.
Truesight mobs will aggro even if you have invisible. This is regardless of whether you have sneak or not. There is a sound equivalent, Alla doesn't seem to acknowledge it but I call it truesound. Monsters with this will aggro through sneak, but they will not aggro by sight, whether you're invisible or not.
Just to finish clearing this steaming pile of misinformation up: These monsters, and the Taurus family are truesight only. You can walk up behind them and pull on their tails without aggro. Just hope they don't suddenly turn around and see you. All Quadavs detect by sound, yes. But that doesn't mean all Quadav NMs have truesound >_> In fact I'm not aware of any that do. There might be a few exceptions but generally, NMs aggro the same way as their placeholders. Empty have truesound. They are mines in the proverbial minefield. They may or may not have truesight (probably not) but it doesn't really matter, if you're in their line of sight and get too close they will aggro by truesound anyway.
With what you claim in your signature you should really know all this by now >_>