Judy426, I do not believe the Hadcock 228 is a drop in replacement for the rega.
I own a hadcock 228 and it was not a drop in replacement for a modded rb250 I replaced on Michell Gyro SE. (On the Michell, I had to reorient armboard so the cutout was closer to the spindle.) The pivot to spindle distances are different for the rega (222 mm) vs the Hadcock 228 (212.8 mm). Even though you can move the hadcock closer to the spindle in a rega cutout because the hadcock base is smaller, the pivot will be too far away from the spindle, by at least 5 mm by my reckoning. For that reason, even though the hadcock 228 can physically fit into the rega cutout, the 228 is not a drop in replacement for the rega, IMO.
Check out the spreadsheet at www.enjoytthemusic.com, and you can see that the 228 can not be aligned using baerwald, when mounted at 217 mm pivot to stylus distance, using 23 degree offset, 228.6 mm effective length and resultant 10.8 mm overhang.
Actually, the 242 is closer to a drop in replacement candidate, with a spindle to pivot distance of 226 mm. If you can offset the base of the 242 in the rega cutout by 4 mm, the 242 is a perfect drop in replacement for the rb300.
I am using the hadcock with an ortofon 540 mkII, and the 540 sounds much better on the hadcock than on the rega. I own the newer 228 model with the stainless steel arm wand. Unfortunately I haven't had enough other cartridges on the Hadcock to make a fair comparison.
One negative about the Unify that may have been addressed in later models is the lack of damped cuing. This would preclude usage in my system. Damped cuing is essential, IMO, to minimize wear on a cartridge's suspension.