As someone still getting to grips with the best way to use my FH1500, I found this, within the knowledge base, very helpful. http://line6.com/support/page/kb/_/amplifiers/firehawk-1500/firehawk-volume-controls-and-speaker-modes-r796. It should answer your question about speaker modes and which speakers are engaged.
Not sure how you're connecting your guitar to the FH1500 (direct as variax / git in, monitor inputs with another modeller) but there are other threads on this forum that describe which speakers work by default based on what you're feeding the beast. :)
