The X150.5 is more than enough power for the 2.4's. Don't let anyone tell you otherwise. If you want more power sure go for the X250.8 but its really not necessary. The other thing that Kent at Pass in response to my plan on buying a pair of monoblocks in the XA series, was that the XA series is the wrong speaker for the Thiels, that the X series would be a better choice. Thereby saving me $10,000 or so. Its a great combo. I think eventually I may move on from Thiel. The thing that Pass does for the Thiels is that the amps warm the speaker up considerably, fleshing out the lower midrange and upper bass where I think the Thiels can sound a little thin. The other thing that happens is that very high end is no long harsh at all with the 2.4s. It solved any brightness issue I have for me. And the detail, the specificity of timber and separation of instruments with the Pass is phenomenal. If I move on from the Thiels to something else it would likely be to something like a Vandersteen Quatro CT or even a Vandersteen 5. I also like the idea behind the Paradigm Persona 9H, as well as the Legacy speakers with wavelet. These options would presumably tighten things up top to bottom. Before going that route I might try measuring the room and employing convolution through Roon with the Thiels and see how that sounds.