Thanks for the response. I have been looking at the FE-138ESR driver and thinking about it for the last few weeks. Unfortunately, the last couple of Fostex drivers I have used did not match the spec sheets when I measured the T/S parameters. The spec sheets used to be fairly accurate and I could buy with confidence and a design in mind. This has made me a little gun shy on springing for an expensive Fostex driver and getting something unexpected. But they definitely have my interest so I am waiting and reading the forums to see if more information becomes available.