I think there is a way to make a low offer without it being insulting. Something along the lines of:
"Hi I've always wanted preamp X your unit looks perfect. Sadly I have limited funds if you would ever consider accepting $xxxxx please let me know and apologies if I've wasted your time"
"Hi I've always wanted preamp X your unit looks perfect. Sadly I have limited funds if you would ever consider accepting $xxxxx please let me know and apologies if I've wasted your time"