There is no HD connection - so any benefit, aside from the content available and the storage, is in the better quality of the tuner/decoders.
Think of a seperates hi-fi system . A decent amp and speakers can be let down by a poor turntable or CD player. So improving the source, via the same amp and speakers, improves the listen. It follows that digiboxes, viewed with a decent screen, can be just as variable. Otherwise a crappy £20 freeview STB would be no better/worse than a £90 humax or topfield.
So changing an older SKY+ box for a new SKY HD one is, imo, worthwhile - albeit your not getting the full HD benefit. Like a seperates system, the avant will benefit from a higher quality source signal.
Not considering this upgrade to the tuner and storage media - which is what the question was about - just because a CRT is non HD is rather like saying all CD players are digital, and therefore they all sound the same, so don't change.
I have swapped a SKY + for a SKY HD box via a CRT, and the difference was clear, so I suggest its worth doing. The boxes are now a nominal amount - well under £100 - and the bbc hd content is a bonus - like a BBC greatest hits channel. If the box was still £300 or more, then maybe not.
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