6700 Lemon

You don't say what the radio is doing, or not doing. It seems like you're saying the powered speaker output and the headphone output are showing the same symptoms. Is it always, or intermittent? It's always possible to get a lemon with any product.

I have experienced an intermittent issue with the phone out jack on my 6400 and tracked it down to a poorly made cable. I made the cable - poor QC. Anyway, I fixed it by making a new cable with a new connector. Just a thought and apologies if I'm being redundant. I don't think this is your problem but thought it worth mentioning.

I also found an issue with one of the powered speakers that appeared as a loss of a channel regardless of the slider position.

Years ago I had an issue with the jack coming loose from the circuit board on my 5000 because the design at the time did not have the jack mounted to the cabinet. Against all common sense I opened it up and repaired the solder connection. The 6xxx is mechanically different, so this would seem unlikely, but maybe. I would not suggest ever taking an iron to a 6xxx.

I do recall reading a cautionary note about accidentally grounding one of the audio channels. This may be in the hardware manual. Something about using a mono plug in the stereo jack.

The experience with my 6400 showed me how they can sometimes resolve mystery causes. I had one, well documented and with video and worked with Flex for a few weeks. The engineers could not replicate the issue and in the end they swapped out my 6400, which was months old. It didn't have the feeling of being a lemon - just an odd issue. The key was keeping very good records of everything and an accurate record of the setup of the radio at the time of the issue, but maybe this applies more to other than sound issues you're experiencing.

With specifics, I've gotten useful feedback from this community. Call Flex up about warranty matters. Maybe it is covered.