I was somewhat in your shoes recently - local shop carries ARC, Leben, Shindo, LM, Rogue, others. Went there to hear Rogue stuff, and incidentally they also had a used mint ARC VSI55 on hand. I did listen to the LM, and it sounded great, but to my ears the ARC sounded just as good (through a set of Proac D2). The LM stuff is gorgeous, but the VSI55 was priced right and too good to pass up, so I got that and the Proacs. I would still love to own an LM amp. I can't say that one amp would sound better than the other, too many variables...different rooms, different source, different speakers. What happens in my listening space may not happen in yours, and vice versa. My over-riding sentiment for you is this: everything you're listening to, or considering, is great equipment. There isn't a dog in the bunch. So I have to think that there's a very happy ending in your immediate future, whether you go LM or ARC.
Side note - I also heard the LM 502CA DAC and loved it. That's on my want list. But I found it odd that the LM website doesn't even include that model, no specs, driver download, etc. Not a big deal, but odd for equipment at this price level.
Side note 2 - Maybe I'm quirky in this way, but the fact that the LM only comes with a 1 year warranty bugged me a little. We're talking about amps in the $4K-5K range. One year seems short to me. But I still would like to own one, and may still buy the 502CA dac.