ZSE Controller described a MVA loss of separation event when failing to hear the read back of an incorrect assigned altitude.

Narrative

I was working a normally combined sector consisting of 18/8/9. The traffic in this sector fluctuates often being very minimal to heavy for a short period; then back to very light. Today seemed to be that way. There was only one abnormal operation a B737 operating in a track that was not a big consumer of attention. I was in a heavy period when H25B checked on with requests for the active runway at LWS. I called the Tower; ascertained the active and possibility of a Visual Approach. The Tower answered Runway 08 in use no possibility of a Visual. I relayed this to the pilot and asked for his report of weather at LWS and his approach request. He answered he would like the RNAV Y Runway 08 approach. I gave him a clearance of direct EVOYU the IF for that approach. Later he asked for lower and I gave him an altitude of 090 only 600 FT above the highest MIA. As he approached the IF I vectored him 10L to achieve the required less than 90 degree turn on for an IF on a GPS approach. He asked for lower concerned with some requirement on the approach profile. I gave him 084 the MIA. Once the aircraft was firmly in the 080 area I assigned 080; in my head. I actually said 040 and he read back 040; but I did not catch the read back. In my mind I was already moving on the calling LWS Tower for the inbound. I did so and got back to the H25B to tell him to expect 070 in 30 seconds. There was a pause and he said he would be at 070 in 30 seconds. As the pause happened my eyes focused on the Mode C which had just dropped below 080. I said that he should be maintaining 080 to which he answered that I just gave 040. I took action and said 'negative; climb and maintain eight thousand!' by that time he had entered a 070 MIA he had climbed back to 070. At that time I had him maintain 070 until EVOYU and cleared back to EVOYU; cleared for the RNAV Y Runway 08 approach also asked him to report established on approach and after he was; switch to Tower. It appeared to me that he cut the corner of a 080 MIA and dipped 300 FT into a 070 MIA the lowest below an MIA being about 700 FT below the 080 MIA and exiting the side of it as he entered the 070 section. In summary; the error occurred because my mouth said something very different from what my head was thinking. Compounded by my either not hearing the read back; or hearing what I wanted to hear. I feel I did a good job identifying and reacting to the situation after the fact. Recommendation; just having this event occurred has taught me the importance of hear back read back; and although I was not caviler about it before it has certainly called my attention to that fact that yes this can happen to me. I'm not sure there is much I could do here to prevent it since it was not something I intended to happen. However; I will be more aware of this in the future. One thing I could have done better in this case is asked for a D-Side for those flare ups that could last longer than I might think.