I can comment on some of the equipments:
I own R28 (1st Gen), Di20He, R8He and HE9
R28 + Di20He is a sizable jump from R28. This is going from good to better.
R28 + Di20He to Di20He+R8He is a big jump. This is going from better to best.
The above are connected to a speaker system. My headphone system (Focal Clear and Senn HD800s) may not be able to get a sizable difference on the headphone setup between that of R28+Di20He compares to R28+Di20He+He9. However, I find that headphone on Elekit 8900 with Di20He+R8He+He9 is better than headphone on the He9 for headphone listening.
The addition of Di20He to either R28 or R8He is a good increase in sound quality and in my system, is the most significant improvement achievable with that increment of cost.
I don't have R27, so I cannot comment on that one.