I scored a pair of MX880s off amazon for $50 and they're honestly the best value I've found in quality earbuds. Sound quality is superior to the PK1s imho. I did give the MX980s a try, and really only noticed a small sonic improvement over the MX880s, but that may very well be due to the fact that the 980s were a tad too large for a proper fit. The MX880s push the limit in that regard & just barely fit, but the 980s were over the edge.
They're not the most comfortable buds & the volume control is a joke, but sq more than makes up for it.