rehearsal marks

[D Josiah Boothby]

How can I change the height of rehearsal marks? I'm using 2.2.6 and have 
a couple marks which overlap with slurs. One example:

\version "2.2.6"

\header {
   title = "mark overlaps tie"
}

\score {
   \notes \relative c''' {
      \time 4/2
       aes4 es c'2 b bes ~
       \mark \default
       bes4 aes ~ aes8 g f e f1
   }
   \paper { raggedright = ##t }
}