If you check the gallery http://www.lowtek.ca/boguslink/ampro I’ve posted a picture of the fan inside the power supply of my Ampro.
I haven’t been able to get full specs from the web on this fan (2410NL-04W-B30). However, from the avsforum.com posting on fans, someone else with an Ampro had the following fan in their unit (2410ML-04W-B30).
The 2410ML has specs you can find. (4000 rpm, 19cu/ft, static pressure 0.16, 31dBA)
It will be tricky to match specs like that with a quiet fan. Really the best you can do is ~20dBA with a 12cu/ft fan (which would be lower than the stock). It might make a lot more sense to replace it with an 80mm fan. There might be room to fit an 80mm fan in the case, might be tricky drilling the holes to mount it..
As an aside, I’ve been thinking about trying a constant height setup. With the 9″ tubes, I’ve got probably enough light — and a constant height setup would really impress the natives. Something I have to at least try — however, it might not be practical due to headroom issues in the second row.