Finally I have a working system but it's not without issues.
Considering that I am using mostly the same components - drives, PSU, TV tuners, case (admittedly having changed the CPU and motherboard) it's been far far from a pain-free upgrade.
The first problem I found: Asus have put the SATA connectors on the very edge of the board, facing outwards not upwards. It's not their fault, but I had to cut away one of the internal walls in my Antec case to be able to get the SATA cables in. It seems a bit daft making a board that is MATX in size but which will have cables projecting about an inch from the sides.
And with the connectors being so tightly packed, they keep releasing themselves by pressing against each others' release mechanisms, so drives keep disconnecting! I will have to buy some new SATA cables - a pain.
For some reason the Catalyst drivers repeatedly refused to show me the video options properly under "video quality": I've had to re-install them twice to get all the deinterlace options to be visible.
Three times today it's crashed and become unresponsive with the CPU running very hot. I've just disabled hibernation from powercfg, in case it's crashing while trying to hibernate (even though I'd disabled it in power settings.)
Last night the tuners disappeared after a reboot, requiring a power-down to make them come back.
But most annoyingly I am getting stuttering every 10 minutes or so when watching recorded TV. I need more time to diagnose this but it's doing it on HD and SD and it's not a problem on the recording.
Nothing is overclocked, most of everything is set to default settings, and I have a minimal software installation.
On a positive note, power usage is down, by about 40W.
But the issues are really disappointing: for 8 years, across 3 systems and 3 versions, MCE has been virtually bullet-proof for me. This system at only a few days old is far from that. I can really only point a finger at the motherboard and its drivers. As anyone reading this will know, diagnosing and fixing those kind of issues is not fun, so I'm tempted to buy a different board full stop. A reinstall is possible but I am going to have to find a more up-to-date Win 7 disk somewhere. The updates took hours to download and install.
My old system has gone to my parents (they had a my previous, creaking older MCE system) and it's running great, as it always has.
My problem system:
Asus F2A85-M, AMD A10-5800, 8GB RAM
System drive: OCZ Agility 3
Other drives: 2TB Seagate, 3TB WD
Tuners: Pinnacle 3010ix DVB-T & TBS 8922 DVB-S