I happen to play in a band that has
two JBL EON G2(15") powered speakers,
and one Mackie SRM 450 (12"). The Mackie,
against my recommendations(but the singer
owns the PA), is used as a monitor; and
the two Eons as the Mains.
Let me tell you that the Mackie kicks the
Eon G2's. Just more presence, clarity, and
punch. In the case of the Mackie, 12" is
actually better than the 15" JBLs.
I do have a question- would a Behringer
PMX2000 with two passive Mackie 12" speakers
sound inferior to two powered SRM450s?

I would go with the SRM450's myself. Each SRM450 has 400W RMS of power while the PMX2000 has a 'total' of 500W. With 2 SRM 450's that gives you a total of "800W" RMS output power. Plus you'll have less gear to lug around too. The Behringer PMX2000 plus passive speakers would be a less expensive option though. The PMX plus 2 Mackie C300's would be a few hundred less than 2 SRM450's but like I said, you would be lugging more gear around. That's a good enough reason in itself to go with the SRM450's.

Best regards,
Mike

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Roy-Andrè