Help, Boss SD-1 not working, no gain.

I put a true bypass kit into my Boss SD-1 and it worked great. I then removed C6 and replaced some capacitors and a resistor. All worked fine. I replaced D4 with a red LED. This was perfect for me and I should have stopped there. It all worked great.
Then I decided to put the D4 LED and the original diode on a SPDT switch. This worked, however when I turned the gain knob up higher than !/2 way I got a high pitched squeal, not feedback, something electrical. I went ahead and decided to put D6 on a SPDT switch that would switch between the original D6 diode and a jumper.
Now, when I turn on the pedal the volume drops and there is no gain. The gain knob does nothing. The level and tone pots work ok. I have removed the switches and reinstalled D6. D4 is a new red LED. Still does the same thing, no gain, low volume. I checked D6 with a multimeter, seems OK. Checked the LED, seems Ok. Checked for shorts everywhere. Could I have damaged the op amp? Why no gain?

I forgot to mention that this is one of those Boss SD-1's that went for about $30.00 new from Musicians Friend a while back. Were there any problems with those that anyone has heard about? If anyone has any ideas about what to check or replace, I'll give it a go. Thanks.

I went and checked voltages as per a schematic I found on the internet. As you know the internet is always right. The voltages check out. So, the tone and level knobs work. The volume is way lower than normal, but the level knob will bring up the volume, but not much. With the level full up, still lower than normal. The tone seems to work. Big problem is no drive. Drive knob does nothing. In this type of situation, (no drive) what would you suspect? I thought maybe the 4558 chip, but someone told me that when they go, it's usually no sound at all. ???????????????????

You might have damaged a wire between the board and the drive pot or possibly the solder pad for D4. You may also have the wrong lead from D4 going to your switch, or your LED oriented incorrectly. You could also check for solder bridges in the diode area. If none of that helps, I guess I'd try to reverse the mod to the last version that worked and go from there.

Thanks cousinpaul, I'm going to check those things out!

Check to see if one of the lugs on the pot is not touching the case while you're in there

I went around and touched up solder joints and checked connections. I found that the pads where the components are soldered are super thin and some were not connected to the traces. I sanded off some of the trace and soldered the components to the trace itself. I now have all three knobs working and I have gain (drive)!
However, it sounds like there are no highs getting through. Like someone turned the tone knob to zero on the guitar. True bypass works fine and the tone knob on the boss works, but no higher tones. Maybe a cap not connected somewhere?

Congrats on getting the gain back. Stay with it, you'll figure it out.