The best replacement for 6550 is kt88.

The best alternatives to 6550 are rated as following:
★★★★★  kt88
★★★★  el34
★★★★  6l6
★★★  kt66
★★  5881
★★  kt120
★★  el34 tubes
★  kt88 tubes
★  kt77
★  6530
★  kt90
★  6l6gc
★  kt88 tube
★  5650
★  6550a
  6ca7
  el34 jcm 800
  kt 88

Your alternative for 6550 is not listed? Suggest!
Speaking about 6550, one probably shouldn't forget about el34.

Other search terms related to 6550 and their ratings are:
★★★★★ el34
★★★★★ 6l6
★★★★ kt88
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6550
kt88
el34