There is a brand of fans called Vornado which run extremely quiet. Might be worth a try.
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Are you sure that your amp is getting overheat? I know that for SS amp getting overheat isn't a great idea since paramenters of transistors will significantly change. I don't recall the part number, but if you do a thorough research on www.partsexpress.com you will find very inexpencive such named amplfier cooler that you can attach to the place where the output transistors are located. The noise will not be an issue in that case.
Your large fan might be a bit overkill & should not be blowing cold air onto the equiment anyway; orient the fan to draw heat away from the amp which will keep it much cleaner (you're presently packing dust into your amp). I use 4" muffin fans & a variac to slow them down to an accepably quiet level. Look for the quieter DBA fans that will still operate at adequate airflow volumes.
Try using a dc fan with an ac/dc adapter preferable a ac/dc adapter with diferrent voltage settings. This way you can select a lower voltage for quieter operation at lower volumes. Plug the ac/dc adapter into your switched outlet on your preamp , if it has one. Make sure the fan is pulling air from the heat sinks rather than blowing into them. Radio Shack has theses items and are inexpensive.
For those who think that taking air away from inside the amp is different than blowing air into the amp, the air that you are drawing away (induced air flow)is replaced by air going into the amp and this air isn't any cleaner than the air blown (forced air)directly into the amp. At any rate, you should simply vent the area surrounding the amp and avoid inducing or forcing air into the amp. This would apply to tubes also.
You want a "Whisper Fan" which is the brand name for quiet, slow fans from Rotron... designed for this sort of application.
As noted, your amp should be free of obstructions - not in a cabinet or other enclosure, and be able to freely circulate air by natural convection. It may even help to lift the amp up off the surface it is now on by a few inches or so...
doing this may eliminate the need for forced air cooling.
But, having said that, unless you are in a huge room or have very inefficient speakers, you are listening too loud if you are overheating a 380 watt amp. You may be clipping on peaks which will also cause an increase in heat.
Check your SPLs. Hearing is a valuable thing that once lost does not usually come back, ever.