1. I'd prefer to prioritise recording more, over quality.
2. Avoid issues with only HD being available, by only prioritising SD, rather than requiring SD.
3. Avoid problems with VM having short-term promotions making HD channels available which then disappear making recordings unwatchable.
4. The "Get in HD" recording setting should just have two options: Prefer SD or Prefer HD.
I’m not sure what more you need than setting the series link up on the SD version of a channel.
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I have some sympathy with the third point regarding short term promotions, but otherwise Ernie's spot on - just set your SL for SD channels rather than all-channels.
Where channels are in HD-only, you could try recording on the +1 variant where one is available.