02 Boxster S header/O2 help?

[Sammy]

I recently put on a set of headers which increased the pipe diameter and removed the primary cats on my 2002 Boxster S. The bad news is that after 150-200 miles the CEL came on. I initially installed the O2 sensors in the runners of the new headers (not at the collector) and had a couple bungs welded in past the second set of cats. I extended the wire for the second set of O2 sensors using crimp connectors. I initially thought that the one of the O2 sensors was bad so I replaced it with a Walker sensor. After another 150 miles the CEL came on again. I thought maybe I should move the O2 sensors down to the collector area of the headers so I extended the wires on the sensors and screwed them in. After 150 miles the CEL light came on yet again! The codes are P0133 and P0153 which are aging pre-cat sensors (the MAF code also showed up). Meanwhile I also installed an Agency Power CAI (after the first CEL showed up so I don't think it is the MAF?).

derb helped me looked at what is going on with his Durametric tool and the post-cat sensor on the drivers side wasn't fluctuating its voltage.

Help? Is it possible that I fried the front sensors when I had them installed in the runners of the new headers? Is there something I am missing? It looks like I'll be purchasing some more O2 sensors but sure would like to know what went wrong? I really like the sound of the new headers and believe I have gained some power at higher RPM's but am not going to drop $$$ in a bunch of sensors trying to make it work.

Thoughts?

Sammy