Hi

Yes it will work.

You can simply connect MIDI Out from the HD500 to MIDI In on the M13, ensure that the base MIDI channels are set to the same channel number (default is channel 1) and as you select presets in the HDD500 it will send MIDI Program Change messages which the M13 will receive and interpret (up to program #48 because that's as many scenes as the M13 can store), or you can set the base MIDI channel on the M13 to a different channel than the HD500 and individually program the footswitches 1 - 8 on the HD500 to send messages on any MIDI channel you like which will change scenes on the M13 and which can turn individual M13 FX on and off.

You have the choice of putting your M13 in the FX loop of the HD500 or you can put the HD500 in the FX loop of the M13 (I'd put the M13 in the HD500's FX loop), or you can connect the M13 in such a way as you place some FX before the HD500 and some in the HD500's FX Loop or you could do it the other way round.  This is similar to the 4 cable method of connection that you can use with some amps.

The HD500 uses M series FX anyway so, of the two the HD500 is the more powerful and the more flexible, but there are circumstances whereby if you push it to the limits you can run out of DSP resources on the HD500, and this is where you might find the M13 most useful by allowing it to provide some of the more DSP 'expensive' FX.

Although you can control the M13 by MIDI certainly, I'm more of a one scene type user in general, and I simply have my set of 12 FX grouped as I want them most of the time and I treat the M13 more or less like a set of traditional stomp boxes, and that's the beauty of the M13 because you can keep it as simple or as complex as you like.

You can find out about using the HD500 as a MIDI controller in the HD500 Advanced User Guide in Appendix B: MIDI and specifically on page B-4.  This should link you directly to the HD500 Advanced User Guide http://l6c.scdn.line6.net/data/l/0a06434dc7704ca4b3b26b924/application/pdf/POD%20HD500%20Advanced%20Guide%20(Rev%20F)%20-%20English.pdf which can also be found by clicking Support above and selecting Product Manuals

Nick