recently a technician of a guitar shop told me I will have a clear sound selecting 16 Ohm on the head and using a 8 Ohms cab. After little bit of researches, I decide to ask here. (because you guys are awesome) Is it dangerous for the head? Should I do it? Thank you.
The only thing you definitely should not do with a tube amp is running it without a load. This will destroy the output transformer, because of the same reasoning given below (very high output voltage).
What happens if you connect a cabinet with lower resistance is that at same output power you will have a lower output voltage and a higher output current. That is governed by the so called ohms law.
If e.g. your amp would output 1.1A output current at 17.9V when connected to a 16Ω cab, using a 8Ω would result in about 1.6A at 12.6V.
You see what happens. The output voltage will drop, the output current will increase a bit. Really: 1.6A or 1.1A is nothing that will cause any trouble. The drop in output voltage will result in a loss in mids.
So: If you like the resulting sound: no problem at all.
Some amp manufactures (e.g. Hughes & Kettner) say that the impedance mismatch should at maximum be factor of 4.