Over the last few months I’ve been running a Minimax preamp (siemens and bugle boy tubes) into the S300, works beautifully. Smoothness of tubes with abundant clear power. Great for piano and guitar. For violin and voice I still think the Class A amp has a slight extra sweetness. Since the preamp has dual outs, sometimes I fire up both amps for a best-of-both-worlds experience (or a critic might call it a split-the-baby compromise).