I'd agree that the 6X10 elliptical DD at 6.59 kHz would be a nice addition. But I wouldn't hold my breath on any future coils at this freq. Back in the day, White's had several models that shared this frequency. Classic III, 6000 series, ID series, Quantum series, XLT series and the XLPro. The XLT series is the only one that is still being manufactured, with the XLPro going away in 2005 and the others prior to that. With just one detector model using this frequency, and considering many of their other models share coils, I can't see anyone producing 6.59 coils. For the record, the MXT and DFX were being manufactured at the same time as many of the 6.59 series were being made. So it wasn't like White's didn't have access to the coil shells. I'd guess it just wasn't in their market forecast?
I can't say I've heard of a larger DD coil than the BlueMax shooter. But I know Jimmy Sierra handled some aftermarket coils, as did Kellyco. Neither manufacturing their own. (Big Foot, Hot Foot, Hot Shot, Hot Head, Ferret etc.) I also remember there being a Bounty Hunter series that was something like 6.6 kHz. Some folks found them to work well on their 6.59 kHz units, but I've not tested one myself. It would be a simple solution to make an adapter plug of sorts, to allow the BH coil to work on your White's. Just a matter of matching up the Xmt and Rcv pairs. But again, I can't say whether there is a DD version or not.
If you learn of any that work and are still available, let the rest of us know! Thanks. HH Randy