Firstly – I have noticed that Musical Mode gets switched on by default for loops dragged from Media Bay only if the loop has compatible embedded tempo information, is an Acid file, etc. It won’t happen if you drag a random untagged file with no embedded tempo metadata, regardless of which preview setting you make in Media Bay beforehand. Now it should work properly with all Steinberg-supplied loops, and if not, maybe you are in bug territory
Regarding the Groove Agent One observations. Bear with us here, it’s a deep topic. When you say you applied time stretch to the file and then dragged it into Groove Agent One, do you mean you engaged Musical Mode? I ask because the issue with Musical Mode is that it is a realtime time stretch – in other words the file on disk is not affected, instead the event’s playback rate is stretched to fit the tempo at the actual moment it is played back. You can confirm this by noticing that if you engage Musical Mode to get the loop to conform to the Project, no bounce or mixdown occurs (i.e. no processing progress bar pops up). The stretch will happen only on playback.
One of the main advantages of realtime stretch/compression is that it will conform to the project’s tempo without fussing around. So if you for example decide that after some listening, your global tempo works better at 130 than 128, you don’t have to go back and re-bounce all your loops to be x number of bars at the new tempo. The other advantage is that if you have a gradual tempo change, it will change the tempo of your loops in realtime – which is impossible by any other method than a realtime stretch.
The disadvantage is that since it is a realtime parameter, when you drag and drop that file somewhere else (such as Groove Agent One), it doesn’t respect the settings for the event in the project window and instead just plays back the file as it exists on disk. Honestly it might be helpful if this default behavior changed – but then what happens to the file on the pad in Groove Agent if you change your tempo
What I would suggest for your situation. First disengage Musical Mode for the event in question. Then use the Time Stretch tool (click and hold on the arrow tool in the toolbox, and select “Sizing Applies time Stretch”). Then drag the end of the audio file to be the length it needs to be, and it will create a bounced version that plays at the proper rate. For example if it should be a 2 bar loop, but it is currently something like 1.76434 bars in the Project window (because it is the wrong tempo), use the Time Stretch tool to lengthen it to 2 bars. You’ll then have a tempo-synced 2 bar loop that should work at that tempo when dropped into Groove Agent One.
If you do this, make sure that that the “Time Stretch Tool Algorithm” setting in the Preferences is NOT set to Realtime, or it will defeat the purpose.