I have listened to the MP-3 quite a bit as I know a few people who own them. At CES I sat with Ralph Karsten for about an hour listening to the MP-1 with vinyl as the source (Porcupine Tree no less, Ralph is a big fan). The MP-3 is very transparent and detailed. It packs a lot of punch in the low end, but overall is very tonally balanced and accurate. One of the best tube preamps for rock music IMO, but I've enjoyed listening to all types of music with the MP-3. The MP-1 stretches it even further, but considering the price difference I really think you could be happy with an MP-3.
Please note that each time I've listened to an Atma-sphere preamp it has been mated with an Atma-sphere amp. I think this contributed to the overall sound I heard.