Static marshall watt - Behringer XENYX 2442FX Mixer

Before removing tube, check the input connection inside the amp.

Output transistors, or output transformer.

Often the boards will be the same, but the power transformer MAY have higher voltage. Also you have to be careful as the speaker might NOT take more power. If you need more power, get a bigger amp. A lot of the smaller amps use the TDA2050 chip which can generate a max of 32 Watts RMS. The bigger Marshall's 50DFX use the TDA7293 or a pair of them which is a different animal entirely.

yes

Check for a dry/cracked solder joint where the 1/4 inch phono input jack connects to the printed circuit board. Solid state Marshalls are renowned for this.

Yes you can as long as the speaker impedance when paralleled doesn't go below the rating of the amp. Marshall seems to hide their design lowest output impedance. Possibly you can find that in your owner's manual. Then find the impedance of the JBL. If half the JBL impedance is equal to or greater than the lowest impedance the Marshall is rated, you are fine to use it. The speaker Watt rating should always be bigger than the amp rating which it is in this case.

They were designed to be used with the Marshall MG100HDFX heads.

Almost all of these small 15 Watt units run 8 ohm speakers...

Do NOT expect to get much improvement from changing speakers.

The value of r32 on the g15r cd amp is 10 Ohms 1/4 watt Brown-Black-Black

I suspect the power supply for the effects processor has failed. The effects unit will "go crazy" when power supply is too low voltage.

If the amp has loud noise, then the power amp portion is PROBABLY OK.

I had a similar problem in a Behringer amp caused by a bad electrolytic filter cap.