The best replacement for 6505 is 6505+.

The best alternatives to 6505 are rated as following:
★★★★★  6505+
★★★★★  dual rectifier
★★★★★  5150
★★★★  6534
★★★★  valveking
★★★  jsx
★★★  3120
★★  jcm 800
★★  5150 iii
★★  6505 plus
★★  triple rectifier
★★  engl
★  5150 ii
★  rectifier
★  6505
★  6262
★  engl powerball
★  ironheart
★  jvm
★  evh 5150
★  orange rockerverb
★  marshall jvm
  mesa dual rectifier
  krank
  engl fireball
  evh 5150 iii
  vypyr
  fireball
  mesa boogie rectifier
  carvin v3
  dual rec

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Speaking about 6505, one probably shouldn't forget about attenuator.

Other search terms related to 6505 and their ratings are:
★★★★★ attenuator
★ tube screamer
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Google Trends

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6505
6505+
dual rectifier