Variac Transformer for speed control?

I have an Elicent 6 fan, 309 cfm that I considering as I upgrade my grow area. Currently I am using a 250hps but moving up to 400. But my problem is noise. I MUST have stealth!!! I grow in a closet in a spare bedroom thats been modified to vent into the attic and out through the roof. I have been researching quieting this fan for a while now. Ive read about building boxes with foam in them, I purchased a muffler for it, i know about insulated ducting. Ive been at the growing game since 2004 so I have a few grows under my belt. Ok enough said, Now after research it seems that slowing the speed down is a good option. But your standard dimmer style controlers can cause motor hum and create heat. So after reading a few threads, ive discovered the variac transformer for speed control.

Ebay has a few nice options. One is 5 amps for $49 the other is 10 amps for $99. My question is which of these should I use for my elicent 6 fan?

the 5 amp will be fine. i have two different speed controllers and i havent noticed th humming you talk about but using larger fans with reduced speed is the way to go for noise.

I hope this isnt a stupid question, but is the amp rating for the transformer itself or the amp recomedation of what youre running through it?

it should be max amps it can power.