DSL 50 loop issue

I recently acquired a 2004 DSL 50. I'm having an issue with the loop where if you put a pedal in the loop, or even just put a patch cable from send to return, there is a noticeable high-end tinny presence added to the signal. You can even hear it when you turn the volume of your channel off.

I did some research and it is this exact problem described here:
http://gsharp.co.uk/fx_mod.html

They have their own solution, but I fail to see how this could be affecting the whole line as to me it makes the loop unusable. I only use a delay really, but it makes the signal sound tinny even when the pedal is off. I haven't played it loud enough to see if I notice it at band volume, but in the bedroom, it really sucks.

Has anyone ever encountered this before with the DSL loop?

The mod is explained in detail, and addresses the root cause, as opposed to a symptom fix. Correcting the high impedance of that section of the audio path solves the undesired stray pickup and noise from surrounding circuitry. It is the type of mod that ought to be rev'd into the product, by adding another op-amp or going with a quad pack if available. This is likely your best solution.

My DSL 100 has a similar issue but without anything in the loop. After the amp heats up, I can still hear the guitar a bit when the volume is turned to zero. I wonder if this mod would fix that.