First, if you haven't already, download a copy of the Nomad manual, available
here. Next, read the manual, taking time to make sure you really understand what each section is saying. Then, read it again for good measure.
If you're hearing buzzing or some other kind of noise, first check to make sure everything is properly grounded and shielded, including the outlet which you plug the amp into, the pickups in your guitar, and the control cavity which houses the controls on your guitar. A lack of proper shielding or grounding can allow noise to find its way in with your signal. Now, if the noise comes only when playing on channel 1 (clean) then V2, may be suspect, as it is the 2nd and 3rd gain stages for the clean channel only. However, you describe the noise as a buzz when playing on the E string through channel 1 (clean). Is the buzz only heard on the E string (high or low?) when playing clean, and not when playing with distortion on the 2nd or 3rd channels? If that is the case, I cannot help but wonder if what you're hearing is fret buzz, which might point to the guitar's setup needing tweaking.
With regard to tubes, there is a diagram in the back of the manual which explains what tubes do what. You'll need 5 preamp tubes of the 12AX7(A)/ECC83 type, and 4 6l6GCs or EL34s. As in the past, there are many different brands and models of 12AX7 tubes available today, and many are of decent quality. Each different brand or model will have a slightly different sound, and so mix and matching different brands of preamp tubes allows you to tweak and tailor the nuances of the amp's voicing to your tastes. The tubes in the preamp don't have to be the same as each other, nor the same as the tubes in the power amp, but the tubes in the power amp will have to match each other closely for the amp to work correctly. The preamp tubes will have more effect on your tone than the power tubes. The coloration the power tubes add is very subtle until the amp is turned up very loud.
You can go with either current production tubes, which, while being made primarily in Russia and China, are decent and will work in your amp, or you can use old European and American production tubes. The old production tubes (commonly and sometimes incorrectly referred to as NOS), while being more expensive and harder to find at times, can provide tonal flavors that the tubes made today cannot. If you're willing to spend the time and money hunting down deals on old tubes, you could take the time to compare new and old production back to back and decide which you like the sounds of better.
