I would personally accept nothing less than an S9 Autoset.
Repeat after me, if you can't dispense a Resmed S9 Autoset with an H5i humidifier I will have to find a DME who can.
As has already been stated they use the same billing code and the same payment to the DME, so the better more expensive machine means less profit to the DME.that is their problem it should not be yours.
Stand your ground even if you have to change DMEs. Also any doctor or dentist can write your prescription. If necessary shop the doctors you know, even your neighbors to get the prescription you want.