Have you tried other speakers to see if it's your preamp/amp? It could be a bad tube, or maybe some pots that need cleaned.
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You've swapped um so you know it isnt your amp. You've run a mono source so you know the inputs are equal. Your old speakers had the same volume in the same positions, so it isn't the room. Capacitor/resistor would be in xover I guess so is it part of the frequency range that's louder in one, or the whole spectrum? No buzzing in the softer speaker? Those are all the things I can think of.. good luck and hopefully someone can supply a definitive answer for you.