I replaced my VP150 with a M60 about 6 months ago and I'm extrememly happy with the results. However, I do not have the speaker housed in a cabinet. It just sit upright between my two mains under my plasma. Thus, I cannot comment on how the speaker will perform inside a cabinet.
Most people cannot use a vertical center because of their configuration (television, furniture, etc.). However, if this isn't an issue for you, I strongly recommend using a M60 for your center, creating a perfectly seamless front soundstage.
