Amp vs preamp question

If you are asking that question, buy an integrated. :)

Douglas Schroeder has a good point.

You want to be sure what you buy synergizes well. If you are not sure what will, an integrated would be a safe bet. Just make sure the amp can drive the speakers adequately.

Another option is trial and error. Buy used and don't overpay if that is the way you want to go, then if something doesn't work out you can sell and buy something else without taking a loss (who needs more of those these days)?

Very few integrateds have the power, especially into sub 3 Ohm loads, to get the most out of the Thiels.

IMO - Amp/Speakers matching first.
Finding preamp that will complement the amp/speakers synergistic qualities is the trick. Transparency is the goal and prefered coloration - just like salt & pepper- to ones own taste. Gamma of features, ease of use, cosmetics, value - all, just iceing on the cake.
At the end, your system is or will be as good as your weakest link. Speaker>amp>pre>source>rest is my own general rule.

Unsound, good point. If he's driving the Thiels to high levels in an 18x18 room, he's got more problems than amp power; he'll be working on hearing loss. 4 Ohm nominal impedance, 90dB sensitivity. He'll be fine. I have used integrateds (i.e. Pathos Classic One MkIII, Eastern Electric M520, Dussun V8i) at moderate listening levels with less efficient speakers and they all have worked perfectly.

Now, if he wants to blow the walls out... that's a different story. Most audiophiles I know do play their gear louder than I do, so take that into consideration.

True, he can get more power with separates, but likely more cost and potentially more mismatches in gear. It's a trade off. He may want to stick with a one brand solution then.