It is correct the bass output should be 10dB lower, but something at the other end needs to compensate for it, which in this case doesn't appear to be happening.

If you play the same disc's test tones through the HD621 or SPDIF you don't get the 10dB drop - (provided you've calibrated your Meridian properly).

Say when you play the tones you get 75dB on your meter. If you play the disc through a digital input you get 75dB through your sub at the listening position on the bass channel. That is no different from a mixing engineer setting the bass levels to 75dB when mixing a soundtrack. From the searching I did this evening other manufacturers boost the bass channel at the input and it appears that Meridian don't.

I might be missing something and making a complete fool of myself, it just doesn't seem right to me that the same test tones played through three different transport methods don't end up with the same result at the speakers.

If we were just talking about the player's built in tones I'd understand exactly where you are coming from but we aren't we're talking about a disc mastered so that on a properly calibrated system pink noise sounds the same through each channel. That's what the Avia disk is set to measure. Don't forget BD players are by definition backwards compatible - would the output on the bass channel on the DVD of helicopters strafing the village in Apocalypse Now sound 10dB lower because you were connected through 7.1 analogue? Play the same sequence though analogue and then through SP-DIF, and you'll hear a difference in levels. Someone won't have sneaked into your house and remastered the back catalogue of DVDs you bought in the last ten years.

At least I hope you will otherwise my brain has turned to jelly(o)

Has it only been 10 years? What are we going to be watching in 2019?

Tim