I have listened to both the mk 2 and mk3 versions and I have owned the mk3. Stay away from the mk1. The mk 2 and mk3 sound a little bit different. But I would not say there is a big difference. The mk3 is a little more detailed but maybe not quite as lush. The mk2 uses 6922 tubes while the mk3 uses 6h30 tubes. My choice was the mk3. But both the VT100 mk2 and mk3 are great amps and pretty much a bargain at the price the used ones are going for.