What you are essentially doing is running two M80’s in parallel off of one amp. There’s no way the 4308 will carry that load (2 ohm), which is why it blew. For now, run one set of M80’s with the loaner and don’t get crazy with the volume control. If you really want or need to run twin M80’s in parallel, you should pick up an outboard two channel amp that will support the load. Use the 4308 as a pre/pro for the R/L channels. Be prepared to shell out some serious coin for an amp that will drive a 2 ohm load.
"......The wisdom of life consists in the elimination of nonessentials." ~ Lin Yutang