For the output signals of both amps to be the same level, the input sensitivity of both amps must be the same and the gains must match also. Other characteristics also factor into this, but these two are the important factors. If the gains are the same, but the input sensitivity is different, then the output levels will not match for the same signal input. You "May" hear a gap or hole between the upper registers and the lower bass. Borrow an amp from a friend or dealer to use for a week or so before buying one to see if this is something you want to do or if another amp will work. An active crossover works wonders in this circumstance, and they have input matching or gain controls also.
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