I would pick the mp2 as last. If you want transormer out then go w/ the psp-2. Jonny does the sx-2 have xfmr out??? There are many sx-2 users who swear by them...likewise the psp-2. There was a big thread on the Oade board...search the archives.
See if the psp-2 owner will let you try it out first.
As for metal detectors...you still have the mics to contend w/.
The Sax rocks. The only thing I don't like about it is the 9v batteries, other than that, it rocks, HTH ...