Well, the squeal sounds like positive feedback caused by your outputs being too close to the preamp side. It seems you fixed this by shortening your leads. As for the pot cases, you should have a ground wire attached to the back. Trusting that the chassis will ground it is not the best, and also the back of the potentiometers are usually just crimped on. This will not always sufficiently ground the pot. It is not good to attach the filter caps ground too far from the main chassis ground. Since the chassis has resistance every fraction of an inch you get away from your main ground will increase noise due to voltage dividing. If you see a reduction of noise by running a jumper from the back of your tone pot to ground, then hard wire it, and try moving your filter ground away from your pots. As for the power tap, the main thing you want to make sure is that your filament voltage is as close to 6.3v as possible. if you lower or raise it too much, you will shorten you tube life drastically, and also you may get more noise. I personally like keeping my tube voltage lower than the recommended specs for any given tube. That being said, I have never seen a professional guitar amp manufacturer that thought the same. Most of them run voltages way over tube specs, and everyone seems to like doing the same. It will shorten tube life also but not as much as you would assume. Many people seem to prefer the sound that this causes. Same as you, I also like running a bleeder resistor off the filter caps, and it should have no effect on the noise. Lastly, a good way to see where your noise is coming from in your amp is to take a guitar cord, shrink wrap the end that is not plugged in, and use the amps input as a signal tracer. this way if you hear a hum, and think that it is coming from a specific component, then you just move the end of your cord closer to the suspected area. If the sound gets louder then you are in the right spot, and find a way to eliminate that sound from affecting your preamp. Remember to shrink wrap the tip so you don’t accidentally dump 300+ volts through your input.