Theta Dreadnut or 3-Plinius 8200P

My suggestion is both sound very good. However, one other option, if your looking into a 3 channel amp, is to look seriously into the 3 channel Pass X150 X 3!!! If you are lookng for a 3 channel amp, you will not find a better sounding one for those speakers!
Anyway, it' a thought.
I'm not sure if your only looking for those brands, but I'd thought I'd suggest other options.
Good luck

I use a Dreadnaught with a Theta Casa-Nova pre. No complaints here. I fear a Classe pre may color the signal a bit (warm). I've owned two Classe pre-amps in the past. I would recommend taking a good hard listen to the Theta Casa-Nova they are going for very cheep right now in the used market.

Sorry never owned a Plinius.

Any comments on the classe CAV-180

I owned a CAV-150 and a Classe 301. Used them both in my H/T system for a good six months. After purchasing the Dreadnaught I dumped them both and never looked back. IT'S THAT GOOD!