Schmalle

High-pitched squeal + hum sounds like a ground issue. The HXFX gets connected to ground via the cable to the amp input (or return). Make sure the amp is grounded. Test the amp's GND continuity between the input ground and power cable ground with a multimeter. Check without the power conditioner. Check in a different room / power outlet. Check with a different amp.

Ratchet?!

Not sure what the XLR outs have to do with the mic in.

I can't comment on what actually caused the "frying of the chip" without knowing the circuit (and mic and cabling). But here is an educated guess of the cause: When you plug in a mic then two the XLR contacts are connecting only roughly simultaneously. It's possible that plugging in a electrostatically charged mic isn't connected to ground for a very short moment and discharging happens partly through the phantom power circuit - it's only a small fraction of a second but the current might be high enough to damage something in the circuit. So taking this hypothesis seriously there is no need to fear when the mic is already plugged in. It seems to be a good rule of thump to put the phantom power off before you plug or unplug a mic.

None of your sentences ended with a question mark.

There is no need for maintenance or inside cleaning in a Stomp. If something breaks either send it to Service Center. If you open it on your own risk loosing warranty. If that doesn't bother you (like me) you can't really repair stuff in it without special equipment because it's tiny SMD parts for the most part. If manage to find a part that is broken you can ask Line6's Support for the parts name and order it from an electronics seller. And you can order a circuit boards via a Service Center and replace them. Line6 doesn't answer in this forum. If you want to address them directly contact their support.

You may get the frequency response right. BUT: listen to the attack. There is a difference in attitude to a bright cap "crushing glass" distortion.

No. Trading latency for a stable sound is working on operating systems like Windows, Mac OS or Linux because other processes can delay the CPU access of the audio process long enough so that audio gaps occur. This is not the case on a real time DSP system like the Helix.

A JCM kicks azz "Sure, we have that amp" they said Wanna kick their azz

IMHO the aggressive bright cap Marshall sound (roaring with crushing glass highs) is more ore less a blind spot in the Helix. Personally I really like the JVM OD1 orange mode and would like to see something like that in the Helix. Also there are rumors that the next update is soon to be released and is likely including a stock 2203 JCM 800. Tip 1: I got pretty close to the JVM OD2 orange model (neutral EQ, Drive 3) with the Badonk with Drive 4.5, B 2.5, M 2.5, T 6.8, P 7.5, Master 4, Depth 5, Sag 7.6. Tip 2:The shelving eq in front of amps works to a degree. Boost high freq at 1.5kHz as a starting point.

Why do you use Helix's Send instead of the Output? That would be the simpler thing to do.

The Send output jack has three pins: Tip, ring, sleeve. SendL sends a mono mixed signal via the Send output's tip, no signal on the ring. SendL/R sends a stereo signal via the Send output using tip for L and ring for R. If you connect to your monitor with a TS cable you get the reverb mixed to mono with SendL and the left reverb channel with SendL/R. If you use mono reverb there should be no difference btw. If you use a TRS cable into a balanced input of the monitor it'll sound weird with the SendL/R unless you use Type Mono and Pan L100.

If you have a healthy guitar the Placater Dirty can get you the amp sound without pedals. Keep the master down to keep things crisp and open. I personally found bias at 5.3 to sound best to my ears. The important thing is the right cab / mic combination to match the recording. I have some favs but I don't wanna spoil your fun. Put a looper first in the chain, play and record a riff and then try to match to the album sound. Don't forget to match the room/ambience for the right feel, too.

I forgot about audio isolators as a practical solution. I can recommend the Behringer HD400 for this.

I'd put thin adhesive tape (i.e. TESA tape) around the flat USB cable plug's metal housing so that it is isolated and not contacting the USB socket. Also some small thin stripe of tape on the left pin (GND). Alternatively you can physically cut shield and GND. Why would that help? - You only need USB's D-, D+, 5V connected. The GND and Shield don't need to be connected because GND is already connected via the power plug. The second GND connection (aka ground loop) via USB causes the problems.