I own the Kestral HR's and used them for a while with a Fi 2a3 - 3.5 watts. They will give you a taste of whats going on with SEts but the kestrals really need a minumum of 8 watts, and the truth is probably more.
There are some kits out there for that kind of money or less, and I believe ASL makes some 300b's in that price range (?).
I've kept the kestrals, but currently use the Fi with a single driver speaker with much higher sensitivity.