JMK PCBs has this pretty killer project for an op-amp based Big Muff like they used to make for a time in the 80s. It sounds quite similar in character to a regular transistor based Big Muff, but with a definitely distinct personality. It's hard for me to put my finger on how exactly, since I have a few Muffs and different builds of Muffs and they all sound different. It's easy to get lost in the sound and just get carried away playing. Big Muffs are meant to be played. Buy this one from me and play it. You shan't regret it, friend!