What speaker cable for Martin Logan motion series (AMT) tweeter.

Given you haven’t found the right cable yet I’ll reiterate my earlier recommendation for the used AZ Satori shotgun cables for $850 that still seem to be available.  If they don’t work out you can very likely sell them for no loss so very little risk, and I think there’s a good chance they’ll do everything you’re looking for.

If you bi-wire I’d highly recommend these AZ shotgun cables that are in your price range.  I have them and they’re nicely detailed but not bright at all with great tone and 3D soundstage abilities, and a few people here have replaced Mogami with these and have been very happy.  Best of luck.

https://www.usaudiomart.com/details/650117484-acoustic-zen-satori-shotgun-biwire-speaker-cable-8-foot-pair/

Depends.  What cables are you using now, and what specific improvements/sound characteristics are you looking for?  Also, what’s your budget and are you looking new or used, and do you want bi-wire or not?