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I have a chance to buy a $50 Onkyo HK-SR503 receiver that has multiple analog inputs for DVD players with surround outputs. Is there any reason those inputs couldn't be used for a multichannel active system.
No reason at all.
Old receivers with 5.1 or 7.1 analog input are a fine choice for low cost active systems.
Try turning off all processing in the receiver. Most will have some processing for setting up the LCR and surelrounds for full range or small (bass cut off) and individual channel volume setup to account for distance, fancier models may have delay.
I figured as much. I've heard from the seller except for the volume control on the remote not working it looks good to go. Thanks for the confirmation.