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I've been trying to wrap my mind around impedance/power/cab-matching and figure out how to get this right without breaking anything, but I'm still not 100% sure.

I have an Ashdown ABM C210H 500 (containing 2 10" BlueLine drivers and that can deliver 575W RMS). From what I read, the internal speakers are wired to 8 ohms total, and gets driven at approximately 300 watts at 8ohm. The amp has speaker outputs that require at least 4ohm load. The power amp is solid state and full power is realized into 4ohm.

Would I be able to connect an extension cab (with a load of 8ohm) rated at 650W, in parallel with the internal speaker configuration, without damaging anything? Would I be correct in assuming that both the internal speakers and extension cab will both be driven at around 500W at 4ohm? I assume the internal speakers would be able to handle being driven that high at 4ohm when running in parallel with the extension cab? I haven't been able find any ratings for the internal speakers.

Any advice would be appreciated ?
    Yes you may. You'll end up with 4 ohm total which is fine. Some solid state amps can actually handle 2 ohm. I don't think the wattage of the speakers that you mention is a determining factor.
      Yup ?

      Just don't go lower than 4 ohms!
        Yup. As long as total load is 4 ohms everything will be fine. Internal speakers and external cab are both 8 Ohm, so they'll divide the power between them, each will see under 300W, or 150W per speaker.
          Expect the tone of the amp to change drastically, unless you connect an Ashdown extension cabinet and speakers. If you where used to a specific tone of your amp you will have to make some adjustment on the tone circuit of the amp.
          I recently got a Ashdown MAG300 Bass combo with a single 15" Ashdown bass speaker. When I connect a extension cabinet which contains a 15" EV Speaker(out of a old Fender 2x15"Bass Speaker cabinet) the sound of the amp became much brighter with the same setting on the amp. You will also have to descrease the volume setting as the amp output dubbel the amount of power now as the impedance has changed from 8 to 4 Ohm. The extension cab will have to have speakers that can handle the power output of the amp. So this would be in my case alteast 150W RMS.
          Which speaker extension cab will you be adding ? Do you know which brand/type of speakers it contains ? Together they(the extension speakers) must be able to handle (in your case) atleast 250 RMS (as the output of your amp is 500W RMS at 4 Ohm. This only applies if you would be playing at full volume which I doubt you will. I Would always buy an extension cab that can handle higher power than the amp can deliver and is slightly bigger in volume with decent port vent holes(for bass speakers) to ensure a better overall sound. The best is to take your combo with you to the shop where you intend buying the extension cab and try it out, if they allow you. You will need to test a few to find the correct tone suitable to your playing style/or music. Have fun, you'll find it isn't as easy as you might think.
            Thanks for the replies, everybody.

            Sounds like it'll work fine then.

            @babbalute I really like the tone I have right now, so I want to get the Ashdown ABM 410H extension cab to keep the tone as close as possible to what I have now. Just more of it ?
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