Strange volume problem

[Ripthorn]

So I finished my last build a month or two ago and have been enjoying it since. The problem is that yesterday morning something happened that I can't find the cause of. On the clean channel, I have a DPDT toggle switch to bypass the tonestack and the output of the tonestack goes through an opamp gain recovery stage. I set the gain recovery stage to give me approximately the same volume level as when the tonestack is bypassed.

So yesterday morning I'm playing and notice that the clean channel (with tonestack bypassed) started to kind of cut out and distort some. The level dropped dramatically. The really strange part is that when I turned on the tonestack, the level was the same as it was prior to the cutting out and distorting. It has stayed like that while the bypassed signal is still very low level and distorted. I have checked the switch, but it seems to be okay. I also adjusted the gain of the gain recovery stage to unity (same level as coming out of the tonestack, so it should be much lower than the bypassed signal) but it is still louder than the bypassed signal and much cleaner (it doesn't have the distortion mentioned above).

In order to give you a better idea of the circuit path, the clean section is a two stage preamp and its output goes to the toggle switch for the tonestack which then goes to the volume control.

I'm sorry if this sounds like a bit of a ramble, but after a long time poking around, audio probing and measuring things with the DMM, nothing seems to be causing it, though it is clearly not right. All voltages cheick out and whatnot. I guess my next step is to check the solder joint for the jumper on the toggle switch. Can a bad solder joint cause something like this?

[selloutrr]

maybe a preamp tube going bad and the opamp is making up the loss when the tone stack is inserted?

[Structo]

Do you think your opamp buffer is still working properly?

Have you swapped known good tubes in for the preamp tubes?

[Ripthorn]

I will try swapping out tubes this evening (they are submini's in DIL sockets, so it is a little more involved than a typical tube swap) and report back. Thanks for the suggestion, I hadn't thought of that yet.

[Ripthorn]

Alright, time for an update. I swapped out the tube that I thought could be bad, but there was no change. So I decided to try something that shed some very interesting light on the situation. I took my output to be right after the DC blocking cap of the second stage and played some with the toggle switch (which is "downstream") set to bypass the tone stack. Still the same thin and distorted tone I was trying to get rid of. I then flipped the toggle so that the tonestack was on (but still further down the signal path) and the tone at the decoupling cap all of a sudden returned to being fat and clean, just they way it used to be.

So it appears that the tube is possibly being loaded down when the tonestack is not engaged. The problem I don't think is with the power amp because I am taking the signal from the lineout (switched jack which cuts the signal off from going to the power amp). The only things down the signal path are my relay switching setup, tonestack toggle and volume control. The relay switching allows me to send the signal from the clean preamp out to the tonestack toggle or to the dirty channel input. If I engage the dirty channel, it is still the weak sound, even if the dirty channel tonestack is engaged (which also has a gain recovery stage).

Is it even possible that the preamp could be getting loaded somehow? If so, what could be doing it? There doesn't seem to be much "downstream" that could be doing it, but all signs point to that being the problem.