The Norh isn't especially powerful, and being SET based likely has an appreciable output impedance (the manufaturer provides little information regarding specs or measurements). This suggests that the amp will also function as an equalizer whose cuts and boosts will be tied to the impedance curve of your speakers. The lower the impedance of your speaker, the greater those cuts and boosts will be. Euphonic if you will. If you overdrive the Norh, likely it will generate an appreciable amount of harmonics possibly at the expense of actual power generating the impression that it is louder than it really is.
If your needs are modest in that neither the material or volume isn't demanding, you may find you'll enjoy the unit. Alternatively, you should be able to hook up a Behringer UltraQ in between the amp and preamp sections of your Yamaha for $100 or so. It provides a bypass mode and allows you to dial in the amount of 'tubiness' you like. The Behringer is probably available at local pro audio shops like Guitar Center and I believe if you don't like it you'll be able to return it for a money back deal.
Good luck.
If your needs are modest in that neither the material or volume isn't demanding, you may find you'll enjoy the unit. Alternatively, you should be able to hook up a Behringer UltraQ in between the amp and preamp sections of your Yamaha for $100 or so. It provides a bypass mode and allows you to dial in the amount of 'tubiness' you like. The Behringer is probably available at local pro audio shops like Guitar Center and I believe if you don't like it you'll be able to return it for a money back deal.
Good luck.
