Mike,
Tube pres can be susceptible to hum, no doubt about it, and usually the issue comes back to earthing, EM screening at the tube grids, or power supply noise.
My knowledge of Audio Note is sketchy, but generally transformers can help greatly in eliminating hum as they break earth loops very nicely. I'm pretty sure that design has a highly adequate power supply (Andy Grove design, IIRC, he's VERY good), and very likely includes an inductor, so here are my suggestions:
1. Hum is either 60Hz, mains, in which case it emanates from magnetic coupling of the mains primary circuit, OR 120Hz, when it is coming off the rectified AC on the secondary side of the trafo and just before the filtering. You need to isolate which it is; hum (60), or buzz (120)
2. Place a shorted RCA into the input of the preamp, with all interconnects and the speakers in place, amps on. If the hum disappears, then the issue is further back in the chain, presumably the DAC. Otherwise, if it persists, then it's very likely the preamp, just as you suspect.
3. The input to the first tube in the preamp will be via the grid, naturally, and if you can isolate this connection, short it with a croc clip to the braid connection of the input RCA. This shorts out the input grid without affecting the bias providing, and this is important, the preamp has appreciable gain. We can verify this if you could send me a picture of the schematic, so I can analyse the circuit more effectively. If hum disappears at this point when input tube grid is grounded, then we are talking very likely EMI - induced hum - into the high impedance input point of the tube. This can normally be eliminated by reducing the value of the grid resistor and taking some shielding precautions. Otherwise, keep moving on, and I will talk later about this stuff after you can get back to me (preferably by email, to keep this process private, not boring the others here!!)
Cheers,
Hugh
