Has to be a factory BH-compatible coil, others won't work.
I'm assuming that your machine is equipped with a 7 or 8 inch diameter searchcoil and has a searchcoil connector that allows searchcoils to be changed. There are two different 8 inch round concentrics which perform basically the same and which may give you just a hair more depth than the 7 incher. Then there's a 10 inch round concentric which isn't likely to buy you much if any depth but will give a broader sweep. The 11 inch elliptical DD that was originally developed for the Fisher F4 (and is BH-compatible) will give you a broader sweep and in most cases a little more depth, and it separates targets a lot better than the 10 incher. I haven't priced 'em but the 11 incher probably costs a lot more than the 10 incher, and the 10 incher a lot more than the 8 inchers.
Another approach to "more depth" would be to keep the 101 as your backup and loaner unit (despite its low price it's a good metal detector), and buy a higher performance metal detector. We've got those all the way beyond $1,000 if you're a serious beeperist, but we've also got Bounty Hunter & Fisher & Teknetics models in the $200-500 range that will hold their own against many more expensive detectors under most conditions.
--Dave J.