If you want real midbass you cannot replace 3-ways. 2-way system by design will either be 50/50 on midbass and midrange or the ratio can go either way, but you will never have 100% of both in one speaker. You can get great results with a very good speaker, but that could be costly. It is hard for one speaker to cover a really wide freq range, especially if you want to cross HPF low at around 50-60hz, then your midrange will suffer. But I think you know that already. So, yeah, that 4" set with a dedicated midbass will be great.
And to answer your other question, try keeping the midrange and tweets as close as possible, midbass could be sitting lower. Semi-active is your way to go. Passive on tweets and 4" speakers on one amp, and dedicate one amp for the midbass speakers, which should be designed for midbass duty.
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Last edited by Sasha; 01-02-2009 at 01:45 PM.