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The low in side jack has a lower vol. but should only be1/4 less than the high in jack on most amps, --try this,-- plug guitar in high in then take all the treble off and 1/2 mid. range bring up bass range and if you have equauzation bring down 26k 5k 10k about 2 notches or points. See if that give you better bass tone.
The 12ax7 preamp tubes will act just like that when they get old, in
fact, this would be my 1st check before I go any further, other than
ventilation and heat .Make sure you are using the right impedence output jack to match your cabinet. Make sure the effects mix level is not overriding your guitar signal. Also, make sure that your signal is not being clipped showing the red led's. Also, the variable limiter control, if turned up very high, will do that to a point similar to a compressor/limiter. I would definitely check the tubes for the crackling sound could be just that.
Unplug the inputs to the amp. If hum disappears, then you need to fix the guitar or the cable or keep the guitar from magnetic fields which are present near amps and other powered equipment. Use of Humbucker pickups helps a lot. If amp has objectionable hum with inouts unplugged, take it in for repair.
Realize that the number on the volume control is only relative... it means nothing regarding what the amp can produce. With a high level output guitar you can easily drive the amp into distortion at very low volume control level settings... what counts is the actual output sound volume and specifically the voltage output going to the speaker. Once the amp reaches saturation, any further input just pushes it into distortion or "flat topping".
An oscilloscope on the output will quickly show when amp starts to flat top.
It is likely that the amp is flat topped at the 7 setting. When the amp is producing the maximum voltage output it is capable of, that is ALL you can get out of it... The numbers on the dial mean NOTHING and are only relative. If you have high output guitars you might find the maximum might only be 3 on the dial for example...
If you need it louder, get a bigger amp and speakers. You should ALSO get a sound meter and follow OSHA guidelines for sound exposure... I know several musicians that now are nearly deaf because of being exposed to high sound levels... their music careers are now gone as they can no longer hear well enough. You need to protect your hearing for the long term. Great musicians don't need loud, but quality and inovation in their music.
If you are driving the speaker too hard it will pop. This caused when the voice coil is driven mostly beyond the pole pieces of the magnet. Without the magnetic field the back EMF of the voice coil drops off and the current shoots up. The voice coil can't move farther so there is a discontinuity of movement which causes a pop.
200 watts is a lot to put into a 12 inch speaker so you have to be careful to not drive low frequencies at excessive volume. It is both very hard on the amp and doesn't do the speaker any good either.
If you want loud lows, you should get a subwoofer of at least 15 inch speaker to handle the lows.
Sounds like your master volume is turned up too much. The channel volume will make the squealing go away(caused by high gain and humbuckers). The best thing to do is dial in your favorite presets and change the channel volume to the correct level with the master volume where you usually keep it. I use my Spider Valve at master volume at about noon and the channel volume between nine o'clock and noon. Let me know if this does not solve the problem......
If you can't clean the pots or if you just want to try replacing them, try using
500k pots again, in case one or both of the pots is worn or dirty or
defective. If that doesn't work, and cleaning or replacing the
switches doesn't work, then the problem is probably elsewhere. You could put a 500k resistor (with a sufficiently-high power rating of at least half the power rating (Watts) of the pot) in parallel with the pot, or, to cover all bases, put a 500k resistor from each of the two outer lugs of the pot to the middle lug. That should change it into the equivalent of a 250k pot. You could even just temporarily clip the resistors on, with alligator-clip leads, if you're very careful, just to test it.