2.) Running the power supply at 12V / 5A: outputs 60W and it draws around 190W from the wall. Looks like PSU itself requires about 20W to operate, the rest is inefficiency due to the fact that it is a linear power supply?
It does seem oddly poor efficiency (35%) but its not far off from whats expected from a linear PSU. Unless you have a switching pre-regulator or lots of transformer taps its going to generate heat. You can play with the output voltage and see where the taps change over and where the efficiency drops.
Comparison DP900:
- 24W at idle
- 30V 61W output, 130W from wall = 58%
- 24V 40W output, 92W from wall = 59%
- 12V 10W output, 48W from wall = 42%
Call me crazy but I might not keep it just because of the fan noise.
Don't think its crazy. These instruments are usually targeted at workplace use, where in an office fan noise of this level (I assume) is normal and tolerable. Though some are even too loud for workplace use (rigol dg800p). If you are a hobbyist at home, it may be unacceptably loud. Did you measure dB?
What are the specs of the original fan?
Don't think noctua is an optimal replacement just because its a good brand, it may or may not be a good swap.