<form name="calcForm">
<script>
function izmen(){
chto=document.getElementById("master1").value;
if(chto==1){
document.calcForm.master2.options.length=0;
document.calcForm.master2.options[0]=new Option("Выберите сопротивление", "0", true, false);
document.calcForm.master2.options[1]=new Option("2 Ом", "1", false, false);
document.calcForm.master2.options[2]=new Option("4 Ом", "2", false, false);
document.calcForm.master2.options[3]=new Option("8 Ом", "3", false, false);
// document.calcForm.master2.options[4]=new Option("0,7+0,7 Ом", "4", false, false);
document.calcForm.master2.options[4]=new Option("1+1 Ом", "4", false, false);
document.calcForm.master2.options[5]=new Option("2+2 Ом", "5", false, false);
document.calcForm.master2.options[6]=new Option("4+4 Ом", "6", false, false);
}
if(chto==2){
document.calcForm.master2.options.length=0;
document.calcForm.master2.options[0]=new Option("Выберите сопротивление", "0", true, false);
document.calcForm.master2.options[1]=new Option("2 Ом", "10", false, false);
document.calcForm.master2.options[2]=new Option("4 Ом", "20", false, false);
document.calcForm.master2.options[3]=new Option("8 Ом", "30", false, false);
// document.calcForm.master2.options[4]=new Option("0,7+0,7 Ом", "4", false, false);
document.calcForm.master2.options[4]=new Option("1+1 Ом", "40", false, false);
document.calcForm.master2.options[5]=new Option("2+2 Ом", "50", false, false);
document.calcForm.master2.options[6]=new Option("4+4 Ом", "60", false, false);
}
if(chto==3){
document.calcForm.master2.options.length=0;
document.calcForm.master2.options[0]=new Option("Выберите сопротивление", "0", true, false);
document.calcForm.master2.options[1]=new Option("2 Ом", "100", false, false);
document.calcForm.master2.options[2]=new Option("4 Ом", "200", false, false);
document.calcForm.master2.options[3]=new Option("8 Ом", "300", false, false);
// document.calcForm.master2.options[4]=new Option("0,7+0,7 Ом", "4", false, false);
document.calcForm.master2.options[4]=new Option("1+1 Ом", "400", false, false);
document.calcForm.master2.options[5]=new Option("2+2 Ом", "500", false, false);
document.calcForm.master2.options[6]=new Option("4+4 Ом", "600", false, false);
}
}
</script>
<select name="master1" id="master1" size="1" onchange="izmen()">
<option value="4">Выберите количество динамиков</option>
<option value="1">1 Сабвуфер</option>
<option value="2">2 Сабвуфера</option>
<option value="3">3 Сабвуфера</option>
</select> <br>
<select name="master2" size="1">
<option value="0">Сначала выберите количество динамиков</option>
</select>

</form>