icons - What symbol to use to represent feedback

When reading Which "Like" symbol is best to use I suddenly realised that the UX community may be able to help me solve a little problem:

I'm working on a site which has just gone in to Beta and as part of the beta testing we're allowing the public to send feedback.

The site needed an obvious visual representation to encourage feedback from users. The usual method is the [+] icon. To me this just means 'expand' and doesn't necessarily suggest that the site is looking for feedback. Other options we explored were a pencil writing in a form - this just means edit so I didn't want to go with this. We considered a tick/cross icon and a speech bubble but ended up with a facebook-style "thumbs up"

TLDR - My questions are:

1) What is the best symbol to encourage feedback and why?

2) Does a 'Like' style thumbs up icon either a) encourage positive feedback or b) discourage negative feedback as users will think it is a link to give "Liked" feedback only

EDIT:

The form behind the link is split in to several subsections:

Ease of use

• 2 multiple (4) choice


• 1 sentence answer "How easy to use is [the site]?"

Ease of understanding

• 1 multiple choice


• 1 text answer "How easy to use is [the site]?"

Usefulness for [performing the action of the site] (it's a journey planner)

• 1 multiple choice


• 1 text answer "Please comment on how useful you find [the site] for [the action]?"

Alternative services

• 1 multiple choice

Further comments / improvements

• 1 text answer "Please add any additional comments or improvement ideas below."


• Do you require a response?


• Name (text input)


• Email (text input)

Answer

I would suggest using a megaphone icon, since it is

• easily identified as a metaphor for "speaking out"


• understood independent of age and cultural background of the intended audience


• non-biasing towards positive or negative feedback.