Hi Brian. I was in the same situation as you. My cable co went digital
and sent me a dc730 which castrated my mythtv box. I didn't have any
luck with "irrecord -f" either. So, I took the remote into work and scoped
it up with an photodiode to get all the timings and carrier frequency. Long
story short, I now have a working lirc.conf file that I can use with irsend.
I won't guarantee it'll work with your box but you're welcome to give it a try.