I would think that if the cooler is made of a material that conducts heat better than air (which isn't hard to achieve, and brass certainly does), and it fits snugly against the glass, then it should perform like an amplifier heat-sink and cool the tubes. If however, the fit is not sufficiently snug, any air pocket between the glass and the cooler will serve as a thermal layer. I'm no scientist; it's just my two cents worth.
