Did you hear that?!
Probably not if you're using another preamp.
The first time I listened to my favorite microphones through the MPA1, it was obvious I've never really noticed their full character before. Many pres add a character of their own and frankly, some mics need the help. But, if you have a great sound source and a great microphone, they deserve to be recorded through the MPA1.
By the way, Benchmark service is fantastic. I called them with a question and found myself in a very informative converstion with the engineer who designed the piece. This is a great company with state of the art products.