I replaced the mid-ranges, in a pair of Realistic Optimus T-300 speakers, with these. No modifications, other than sealing gaskets, were made. The mid-range clarity and presence are noticeably better than the originals. My Carver TCM-35 and Holographic pre-amp make it sing. I wish they would have fit my Acoustic Research AR-3As as well, but they would not.
Used these to replace blown GRS 5 in. sealed back midranges, mounting holes were exactly the same, but the ESS sound quality is crystal clear compared to the grainy GRS mids.