| It depends.
Say, you have a 8 ohm cabinet now and you build two cabinets 4 ohms each. Then your amp needs to be able to run at 4 ohm load, but the two new cabinets need to be connected in series (4 ohms + 4 ohms = 8 ohms). These two loads can now be connected to your "output channels" which most likely are just cabinet connectors wired in parallel.
For the series connection, you need to build a "series box".
If, on the contrary, you need for some reason to connect more cabinets in parallel, you need a "parallel box".
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Fender Telecaster Basses & Precision Bass + G & L L-1000 & L-2000 -> Aguilar DB751 -> DB212 cabinets (garage setup) or TH500 -> Laney 2x10 enclosure + Laney NXP 1x15" cabinet (rec room setup)
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