Boosting the sweet eq up in the 12-20k range. This sounds like the Avalon AD 2055 parametric eq, it has a nice 'airy/light' feeling. Also, There is also a piece of equipment which is called the Aphex Aural Exciter type b (type b is the good one) You can find them cheap online, but they are old so the quality is iffy. I bought 3 different ones for about 75 each and tested them in the studio. Kept the best one, sold the other two.

The Aural Exciter is a psycho-acoustic processor and adds upper harmonics to the signal. It is an analog piece though. It will make it sound like you are doing that upper range boosting, but without changing the signal.