Need some advice, Orchids are dying!

[Possum-Pie]

I bought my first Phalaenopsis 5 years ago from a box store. Left it in the original pot, watered about 1/week, and it did wonderfully. A friend gave me 3 more that he didn't want, and they all did wonderfully. I bought 2 more so had 6. A year ago, for Christmas, I got larger ceramic pots from people as gifts. I bought Miracle Grow Orchid media and begun transplanting. I read that some media carried disease/insect eggs, so I microwaved the media to kill any pathogens. After cooling down, I transplanted them all. None did well after that. They slowly lost leaves, and began long slow deaths. 14 months later and I now have only 2 left out of 6. What went wrong? I go to Longwood Gardens frequently, and took an online orchid course by them that suggested that orchids like small pots. Could the pot size have killed them? Perhaps it was the new media...It is medium chunk bark and I put a bit of sphagnum moss. Watering habits didn't change, I began using orchid fertilizer this summer thinking maybe that was the problem, but it didn't help. Symptoms include leaves getting floppy, yellowing, dropping off, crown turning dark, roots shriveling, blackening, then ultimately death. Any suggestions?