You might give it some time and see if it's just some latent tube noise that goes away as they break in. Luke Manley is one of the nicest guys around and I'm sure you could call him and get his take.
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Thanks Atmasphere, and it's true I don't notice anything like this while the amp is running. I wouldn't have cared at all about this probably if both amps exhibited the same behaviour. I switched the tubes from the amp that doesn't have this noise on power off to the amp that does and nothing changed... so this leads me to believe that it's not tube related. I'm not really worried about it, it's just something that I'd like to know why is happening.
I actually payed more attention while powering off the other monoblock (that I didn't think had the noise), and if I put my ear on the tweeter I can notice the same faint whoosh sound but it's not nearly as loud.