That does seem to be the consensus, and after trying it for myself I agree with it. I have the exact same setup as you; TT2 with Diana V2, and also a Ferrum Oor + Hypsos. I bought the Ferrum stack first after I got the headphones. It's definitely a great amp with a ton of power. Then I bought the TT2 intending mostly to use it as a DAC, until I plugged into the front of it for the first time just to try it, and was blown away how good it sounded. MAYBE a little less slam than the Ferrum but really depends on the song; the TT2 has a ton of power.
I've tried many different configurations outputting the TT2 to the Ferrum; RCA, XLR, DAC mode, pre amp mode. None of it seems to make a massive difference in the sound to me, and the best way I can describe it is that you still retain all the detail from the TT2 DAC, but for every other aspect, the Ferrum just dominates the sound; all I really hear is the Ferrum and the magic of the TT2 output is gone. All that unbelievable soundstage and layering is gone, the liquid smooth treble is also gone. Maybe a different amp could be a better pairing, something that colors the sound less, but for me at least, I stopped looking for anything to add after I compared the TT2 output to adding more to the chain.
There is also a consensus that unless you have a 1266 or Susvara, the TT2 is plenty of power and slam for pretty much anything else. I don't have either of these but also something to keep in mind.