ZSE ARTCC Controller reported that complacency led to missing an incorrect altitude readback from an aircraft and resulted in said aircraft descending below the MVA.

Narrative

[I] was working R35. Extremely slow day. Before this; I had gone 20 minutes without a plane in the sector. It was split due to a poor management mandate. The sector was unusually much slower than usual due to poor weather; but Management still had it split. This was my second session in a row where they had staffed chairs" affecting a sector I worked. Earlier; they had staffed a high sector I worked simply because it may go "yellow" based on one of their metrics. There was zero complexity or traffic situations and I was assigned a D-side that did more harm than good as they were not paying attention to the sector at all.It was last session of the day. I got really bad news that day outside of work. All combined; I wasn't as focused as I should have been apparently. I descended Aircraft X to 090 and issued direct AWBAN to set up for approach. They hesitated; [and] read back "Down to six; direct AWBAN for Aircraft X." Perhaps it was the alliteration of the callsign with the altitude. Either way; I missed it. Later; they descended through 090; [and] MSAW triggered. I issued a low altitude alert before the altitude even showed below 090; just based off [the] MSAW. They did not respond. I issued another and eventually with hesitation; they did respond.[Recommendations]; none. They gave a poor readback and I missed it."