Slawek, welcome. Forget what the salesman told you about an "issue" with the crossover to the sub or bass coming from one direction. HT receivers such as your 340 handle the crossover between the speakers and sub routinely with no problem. The bass sent to the sub is essentially omnidirectional and doesn't come from "one direction" when set up correctly.

If you want two subs, you can use a Y-adaptor to feed both of them from the sub out on the 340. Keep thinking of the M80s + EP500.

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Enjoy the music, not the equipment.