...might be considered an absolute must if you own one of these amps. Add's bookoo more low end. Makes the Bass knob actually do something and sweetens tone incredibly. Much more full-range. Still plenty of Marshall mid-range to boot and crystal top end. Already has been my go-to amp of late and now my favorite small combo. Part # MS15-O on the MM website. $127. Well worth the upgrade. Relatively easy to replace if you're experienced with basic amp repair (and safety.) Or shouldn't be expensive if you have a tech do it for you. Easily within one hour's benchtime.