dual voicecoil and 2 amps

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connecting two amps, both americanpro's 400w(170rms) and 600w(255rms) into a dual voicecoil sub rated 500wrms(250 each), while i understand what will happen if the polarities are not connected correctly, cancelling instead of enhancing....
Is there anything that i need to watch out for or need to correct?
i say this cos the sound virtually remains the same when both are connected or just one but is severly reduced when phase is out, why such a very small increase like 0.25 db only and excursion remains the same.
You dont state how you have the amps and voice coils connected... voice coils series or parallel? are the amps bridged? or is each amp feeding it's own voice coil?

Given the power ratings, the single 600W amp can provide more than enough power on it's own... also given that the amps are not identical, they will have different amounts of gain relative to each other so you wont get good results both amps need to be identical for good results.... also, if you wire the 2 voice coils out of phase with each other, you are extremely lucky not to damage or destroy the speaker.
well with double the power, you gain 3dB and you'll gain less than this (assuming the 600W amps is the primary amp and the 400W amp is the secondary amp) because the second voice coil will have less power running thru it because the 400W amp has less closed loop voltage gain than the 600W amp.

what speaker are you using? what is the nominal impedance of the voice coils?
note ive read that when "connecting" two amps to form one amp a sync cable has to be connected between amps to syncronize them, could this be part of the problem, (these amps cant be sync'd this way), also crossover selection should be active but i still need to get that so i'm using the crossovers on the amps.

with what ive got how can i get to feel the 425wrms going into this sub?
excluding box dimensions.....etc
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e96mlo said:
Does the 600 W amp have 255 Wrms per channel? If that is the case then connect one channel to each coil and use the 400 W amp to something else. This should be sufficient since the speaker could handle 250 W/coil.

That is the way i would use it. You have a ton of power and it will sound best if you use it conservatively. Bridging makes each amp in the bridge see half the impedance -- 2 ohms here -- and that is hard on almost any amp.

I get away with 20 W on each of the coils of my HT sub -- you would have some 10 dB more than that.

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