I don't think back to back listening necessarily resolves very much, certainly not with kit that's reasonably matched in terms of price/performance. Time gets consumed identifying differences in presentation and labelling them as 'better' or 'worse'. Too much of this you don't end up any further forward as it becomes an intellectual exercise, with added pressure and confusion.

Put one in for a week without messing around and feel how you respond to it emotionally. Swap it round and do the same. Only compare after you've done this with both how often you did (or didn't) find yourself drawn to just sit and listen to the music, how curious you were to hear how x album sounded vs the last time you listened to it, how much fun you were having. Your guts will tell you the answer if you listen to them.

I've always felt the most satisfied with my conclusions when I've done comparisons this way. Where I haven't done it that way and have flogged stuff off (as I all too often have) to get my mits on the new and the sparkly (occasionally without auditioning first), that's when I end up later buying back stuff for a proper listen to make sure I've done the right thing. I've made quite a few changes of late and just about all of them have been done with careful evaluation over days and weeks, not an afternoon, where either you just end up unsure or (worse) where some characteristic jumps out and makes an impression, but that ultimately isn't the whole story.

Having said all that I'm rather surprised The Grail isn't doing more for you, I don't know the 906 but I had a Trilogy 907 a few years back and it wasn't a keeper, I would have expected The Grail with its LCR design to be streets ahead not just in technical sophisticiation but in what LCR phono stages in the main do beautifully which is fine detail (particularly inner voices or timbral accuracy), spatial cues like 3D placement, room acoustics, and so on. I've been using LCR stages for 6+ years now (Aurorasound Vida, EAR 912 and latterly Allnic H7000V) and have no plans to go back.

BTW I wouldn't expect the Partridge 977 to sound that great with the Benz, it needs a lot more than 1:6 step-up I would think, I'd be looking at 1:12-1:15 upwards.