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Midrange clarity is a priority for me. Playing pop music is not, but I have to share the space and I think we all know how big 604 cabinets are so I want to consider if they would be a downgrade for tastes that include kick and bass pop music. Low end extension can be worked around but I would shy away from them if they were “unbalanced” for lack of a better term.
When I say 604 midrange, it's all about the warmth and intimacy, not about clarity. You would find a better speaker if midrange clarity is your top priority. Also, 604 bass doesn't stop quickly, and this behavior is not preferable for modern pop music arrangement with tight and fast bass with poor overtones. It's not about low frequency extension.
If you're just interested in 604, I would recommend to get a pair. I'm sure you'll know why a lot of people still love it. A lot of limitations, but very musical compromise. It will be neither downgrade nor upgrade since FR and 604 are different animals, and you may end up keeping both. (And I have both)
I think you probably will get a good idea in this following popular thread for the speakers that can play complex music at high volume.
Is it possible to cover the whole spectrum, high spl, low distortion with a 2-way?
If I were you, I would check something like ATC (Large), JBL M2 (large), or Kii (Small), modern speakers that can handle almost anything very well, and their low frequency handling and midrange clarity is absolutely top notch. Those speakers do not sound like 604. Less warmth, more analytic.