I've heard both albeit in different systems. The Sim is more neutral and can control virtually any speaker. The Plinius has that coveted midrange tube lovers rave about. The Plinius has the option of class A operation which does make a difference vs its AB operation. But it gets quite hot in Class A. Keep in mind that the Sim works beautifully with Dynaudio. Consider also the Sim Audio W3. While at only 125 watts (same as the Plinius) it does some things better than the W5. Given that you have a great preamp in the Syrah, I'd give the nod to one of the Sims. You have some good choices here and should consider hearing both if you can.