I use a ST500 on my Thiel CS2.4 and it is a good match. The ST500 plays way out of its price class. The dealer that told you it will not sound good has never heard the combo or is a lier...
The damping factor of the class-d amp is it's real strength. It gives real punch and control to the bass of the 2.4s. I would call the sound very neutral. It is neither bright or warm. The amps highs maybe a touch soft but it is a good balance with thiels 2.4s being a little tipped up in the highs.
I also tried a Mcintosh 252. The 252 had less control in the bass a more romantic midrange and a touch more sparkle in the highs. It is 3 times the price and I like the ST500 better. I listen mostly to rock and value the bass control very highly. Now that I think about it I spend less for my CS2.4 (demos) and ST500 than I would have spent on the just the Mcintosh 252...
The Bel Canto is also a good class-d but not worth the cost difference IMO. It is more different than better. It has softer highs than the ST500 and a little less bass slam but might do the audiophile tricks (sound stage, decay, etc) better.
PS. for what it is worth do not buy the ST1000, you will never need it. I don't even use 1/5 the power the ST500 has with the CS2.4. Buy the ST500 and spend the extra somewhere else.