Polk SL-2500 (c. 1990–1991)

Most of what's known about the Polk SL-2500 comes from Polk's own Monitor Series 2 advertising around 1990-91. It wasn't a complete loudspeaker. It was a 1-inch (25 mm) dome tweeter, and it was the common high-frequency driver across the Monitor Series 2 lineup. A Polk advertisement in Audio (October 1991, p.26) listed it in the Monitor 12 Series 2, the Monitor 7 Series 2, and the compact Monitor 4. Each showed "One 1 inch (25 mm) dome tweeter (Polk SL 2500)" in the driver complement.

Polk positioned the SL-2500 as real engineering, not a budget part. The company's ad copy said it shared much of the technology of the SL 3000 tweeter used in Polk's flagship SRS series, and described the SL-2500 as a highly refined, technically advanced driver (Audio, October 1991, p.23). It also used an ultra-low-viscosity magnetic fluid - ferrofluid - in the voice coil gap. Polk claimed this let it exceed normal listening levels without loss of performance or reliability, with dynamic range that was unusual for the price class.

Beyond those period advertisements, the record is thin. The SL-2500 doesn't appear to have been sold as a standalone retail part, and independent reviews of the driver itself are hard to find. But if you own a Monitor Series 2 speaker from this era, this is the tweeter doing the highs.