You’ll have about a watt of spare power from the THX AAA 789 at 120 dB/SPL, so it should have ample power.
Scratch that, it’s pre-coffee time so I missed a couple of important points …
First, the THX AAA 789 only delivers 3.2W into 60 ohms, not the full 6W. So that’d cap at 118 dB/SPL (not really a meaningful difference, as its well into “stupidly dangerous” territory anyway).
Second, I’m not sure what the voltage rail limits are on the unit - from memory I’m pretty sure it hits at least 16V, and may go higher, but I’d have to go digging through my measurements to be sure.
One of the reasons the Susvara has such a reputation for being a massive power hog is less to do with the headphones (though they are extremely inefficient), and more to do with the fact that most headphone amplifiers that are rated as powerful enough, technically, to hit normal listening levels are pushing up against their limits to do so. Where with most other headphones they’ve still have plenty of headroom and be operating within their sweet-spots.
Amplifiers rarely perform their best anywhere near their limits.