Cary SLI-80 Signature - Tube Recommendation

Hello Audiogoner - Please Advise/Help......THANKS....
Currently, I own a Cary SLI-80 Signature integrated amplifier and would like to try on a new/better tube. My price range is $50-$75 per tube. Please note this is a older model, it's about 6 years old.

1 - Rectifier Tube 5u4g (??????? I have no clue about this)

2 - Input Tube 6922 (I got Amperex white label, any other recommend ??????

3 - Power Tube 6550 - (Genalex Gold Lion KT88 tubes Valves 6550, any other recommend????????????

4 - Output Tube 6sn7gt - (??????? I have no clue about this)

Please help, I would like to hear from you for your recommendation............BIG THANKS.......

Joseph