Best Mullard 5AR4/GZ34?

I'm looking at replacing the rectifier tube in my Modwright LS100 having already replaced the stock 6SN7's with Sophia Electrics. Is there a meaningful sonic difference between the various Mullard rectifier tubes? Should I be looking at NOS or newly manufactured issues? If NOS, which version (and how will I know I'm getting the "right" one) and what should I expect to pay? If new, which of these are the best sounding and most reliable? Advice on where to purchase or even offers from sellers are welcome. Your expertise is much appreciated.