I recently bought a bunch of new pedals and this is how my pedal board look like:

Guitar > Boss CS3-Compressor Sustainer > Big Muff > TC Electronics Spark Booster > TC Electronics Mojo Mojo > Boss GE7-Equalizer > Boss NS-2-Noise Suppressor > Boss RT-20 Rotary > TC Electronics Hall of Fame Reverb > Boss DD-7 Delay. I also have a Boss volume pedal.

I have no problem when turning on some pedals alone, the volume is always the same. The problem is when I turn on the Spark Booster, the muff and some other pedals at the same time. Mostly for my solos. The volume just goes up by a million percent!! I have the volume pedal, so I can lower the volume with it. But I know that's not the solution. I tried lowering the level on the Spark, but it's not giving me as much boost and sustain as it was.

What should I do? What volume should a lower/higher? Any help is appreciated!

Try putting the boost in front of the Muff. The tone will be different but it won't make it that much louder.
Guitar > Booster > Mojomojo > Big Muff > Eq > Comp > Suppressor > Delay > Rotary > Reverb.

That's how I'd go, but switch and swap until you get what you want.

Has your amp got an FX loop?
It's an opinion. It's subjective. And I'm right, anyway.
Yes, my amp's got an effects loop. The Delay and Reverb are going in the back of the amp.