Hello Nick and welcome!
I'm sure some members here that regularly run the M500t will add their feedback for you, but I don't think the M500t would have any problem with the AR-LST. In its stock form it is rated for over 300 watts per channel into 4-ohm loads.
For your reference, the Owners and Service Manuals for this amp are on the site located in the 'Manuals and Specsheets' pulldown at the top of any page.
I don't believe there are any factory service bulletins for the amp (aside from a note about replacement component spacing on the board), but there is a mk II modification that's available. More info here. That mod info is available, and a good service shop should be able to do that work, but I think there may also be some merit in simply using the amp as-is for a while and seeing how you like the performance in your system. Assuming the amp you are looking at is in good electrical condition, it is a very nice performer in its stock form.