The Apple TV works fine connected via HDMI. I use mine connected via a Lumagen XE then on to the HD621. Connecting direct to the HD621 should also work fine.
In terms of getting it all working - you need to use the config program and explicitly tell the HD621 to switch to the correct input upon selecting your chosen input.
It sounds as though you have it connected up OK, you just need to adjust the config and write that back to both the G68 and the HD621 separately (i.e. a two stage operation across the two boxes).
I use a HD621 with a 808v5 rather than a G68, however setting up the system will be similar, I can share my config file with you if that would help.
Where are you in Surrey - there are several of us probably fairly local / can recommend a dealer to you if you prefer?
John
808v6, M60, MC200, HD621, Lumagen XE, 504, SB Touch, 563, 201, 204, 208, MPMAX 221, M80, Explorer, Explorer2.
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