You'll be fine with the 1900 and the M80's. The 1900 is towards the top of the Yamaha receivers and uses full push-pull circuit configuration instead of an all on a chip solution that the less expensive Yamaha model line utilizes.
I believe that is why manufactures such as Denon, Rotel, Onkyo can be blanket recommneded as they never use an all in one amp design for any of their models.
Chris (ClubNeon) should be along shortly to correct me if I'm wrong or expand with explanations if I'm close to right.
I have a RX-V3800 which is an earlier model with a similar design and it has never had a problem with 4ohms.
