The Plinius is a nice choice. Still, even the older Used Threshold T200's are as good and stronger in the bass!...and overall strong enough at 100w even. The Pass X150 is also stellar. I'd chose these later over the Plinius in my experiences with 804's.
A possible better choice for you however is TO NOT SELL YOUR AMP YET! If you picked up another excellent bass amp(like most any of Parasounds offerings, which have adjustable gain controls), you could use that on the bottom, and yours on top, and BI-AMP! This is a MUCH STRONGER combination and option. Before you go "ditchville"
you might consider this. I would bi-amp those speakers, as it seems to be the way to go with them. I've bi-amped those speakers with dual Classe CA151's, and the sound was superb for the money!
Actually, it'll save you money, and possibly improve on bass if you end up with, say a Parasound HCA1500 or 2200 driving the bass. The one thing the Parasound's do well is bass indeed!
Hope this helps