I'm hearing a difference in my tone when I use the Effects loop on my 667. It's a series loop with level in and out controls which I'm still messing with getting set right.

The issue I have is that when I run my signal through the loop, with nothing engaged, I'm losing some clarity and top end to the signal. Definitely noticeable.

I have yet to pull my effects out of the loop and start sniffing where the issue is, but has anyone had an issue with the loop pre-amp tube causing a problem?