Main difference: Transformers. They have different transformers in them. The MKII has more headroom on the clean side and a tiny bit less compression. Easy way to tell them apart, besides looking at the front of the amp where is would say MKII, is by looking at the transformers. In the MKI the transformers are sitting in the amp sideways. In the MKII they are standing straight up. Thats pretty much all you need to know about the differences. Nuances in the tone and feel. MKII is the "improved" version.