>>"I bought them on Ebay. Why do they sound good now?. If it is a bad microphonic tube will it show itself again?"<<
That is just a chance you will have to take.
>>"When I powered the unit up there was a strange sound (as the sound of an AC unit running)"<<
That was noise, could be caused from grid emissions, or a slight intermittent short.
Some times when an old tube lays around for many years strange things can happen when put back into service. If after you get a few more hours on the tube and it remains quiet you may luck out and have a good tube. If it does it again I would pull it and replace it. Shorted tubes can cause damage.
As for the Herbie's HAL-O tubes dampers I use them in my Sonic Frontiers Preamp, and ARC VT50 Power amp. Imo they are great.
In my preamp I have them installed near the top of main body of the tubes.