puzzle

 ACROSS

  1 Protrude, with "out"

  4 Broad neckwear

  9 "C'est la vie"

 13 Greek H

 14 Group that sings hymns

 15 Huge concert venue

 16 Court, knight-style

 17 Late March, say (theme hint: in this answer, switch the last word's second letter with the one below it)

 19 Poem such as "Gilgamesh"

 21 Queen or drone

 22 Consumed

 23 Phoebe Buffay's portrayer on "Friends" (... first word's third letter ...)

 26 Reacts strongly to a sad movie

 30 Kafka's Gregor

 31 What puts the icing on the cake?

 33 Storefront info: Abbr.

 35 Hunk

 36 Aries animal

 37 Egg layer

 40 Perplexed

 42 First half of a pronoun pair

 43 Cheese sharpener

 44 Food with an al pastor style

 45 Best

 47 Iowan

 49 Dashes or sprints

 53 Prefix with "freeze"

 54 Come up with nothing (... last word's fourth letter ...)

 58 Word paired with neither

 60 A state of matter

 61 Without repeating

 62 Chinese regime known for its porcelain vases (... first word's second letter ...)

 67 Currently

 68 Toned down

 69 Verb that sounds like a cow part

 70 Flattering poem

 71 Focusing condition, briefly

 72 Drinks in steins

 73 ___ Talks

DOWN

  1 Gems

  2 Perfect place

  3 Lao-tzu's philosophy

  4 Asexual, for short

  5 When doubled, Japanese hot pot

  6 Prepped, as an apple

  7 Fuel tanker

  8 Sample

  9 Put a halt to

 10 Hawaiian garland

 11 Author ___ M. Martin

 12 Droop

 15 Fitting

 18 Overlooked, as a fault

 20 "Cold, hard" money

 24 Activity that requires chops?

 25 Norwegian capital

 27 Plural possessive pronoun

 28 Uninspiring

 29 "___ time next week?"

 32 Soak up

 34 "Don't get soaked!"

 35 ___-mo

 37 "LMAO"

 38 Susan who played Belle on Broadway

 39 Its eye is in many a witch's brew

 41 PC company

 46 ___ Alto

 48 Crowned, in checkers

 50 Is unable to

 51 Make into a cipher

 52 Slantwise

 55 Striped gem

 56 Compost, e.g.

 57 September flower

 59 Drag queen Vander Von ___

 62 UFC combat style

 63 Birth control option, for short

 64 Indeterminate degree

 65 Short pencil

 66 Century's 100: Abbr.

solution