I thought the NHT Power 5 and Bel Canto S300 all used the same ICE module and both received Stereophile Class "B" recommendation if I remembered correctly. So, it may be a lateral move and may not be much of a step up if any.
I would suggest you look into NuForce (Ref 8.5V2 or 9V2) if you want to step up. I had the Bel Canto Ref 1K for a while until I moved onto NuForce MCH3SE. To my ears, the later is much more transparent, larger and much deeper soundstage, faster, punchier, more resolution and just overall more "real" or "live" sounding. However, the Bel Canto is denser in the midrange but not so much in a more natural way. YMMV.
Kenobi
I would suggest you look into NuForce (Ref 8.5V2 or 9V2) if you want to step up. I had the Bel Canto Ref 1K for a while until I moved onto NuForce MCH3SE. To my ears, the later is much more transparent, larger and much deeper soundstage, faster, punchier, more resolution and just overall more "real" or "live" sounding. However, the Bel Canto is denser in the midrange but not so much in a more natural way. YMMV.
Kenobi