Another vote for the Wally BX42 which I run behind my BX25. I did have a DR PTO
chipper before that which also worked well, but the Wally handles the 2"-4" material much better. It will self feed larger material better than twiggy stuff but you will have to trim large-crotched branches so they feed in. I would not rule out hydraulic feed if you can swing it. I don't know if the BX25 has sufficient flow to meet the spec's for the hydraulic motor on the BX42R, but I"d sure like to try it. Just couldn't swing the extra dinero's for hydraulic feed.
As you can see from the pic's the Wallenstein is much more substantial in size/weight, though the performance of the DR wasn't that much inferior than the BX42. The DR uses a belt and gear to turn the rotor at higher RPM while the Wally is a direct drive that rotates at PTO speed. The DR unit pictured is an older version as they now offer a top side chute for chip ejection, but it doesn't swivel I believe. Anyway, I think you'll be pleased with a Wallenstein unit for your BX25D to be!