How To Fix A Crackly Amp?

My Vox AC-15CC1 sounds great clean with my les paul standard, but when I hook up my Maxon OD-9 for some overdrive, it seems to crackle a little bit. I was looking for a very smooth, almost clean sounding OD/Distortion. This is my first tube amp.

Any ideas, reasons why, or suggestions on how I could just get a nice, smooth, warm & rich sounding OD? Even if that means a new amp or pedal, let me know.

All my cables/ac adapters work fine, and I keep the pedal's drive around 9-12 o' clock.

Thanks guys.