Hello! I recently built two TS500 kits and one of them works flawlessly, while the other passes a very, very dark signal. The meter is accurate, the levels match the other unit that works well, and the bypass works and passes a clean signal. 

When I fist powered the unit up it oscillated like crazy and I discovered I had installed U5 backwards. I ordered a replacement part from Mouser and replaced the chip, and while the oscillation problem is gone it now passes a signal that sounds aggressively low-passed.

I've double-checked all the solder joints and re-flowed any joints that looked suspicious. The problem persists.

Ring any bells? Thanks for your time!