I reworked a couple things I got the subform working the way I wanted. Instead of directly linking the subform to the main form via the asset type, I just changed the recordsource of the form to reference the asset type combo box. The revised database is attached. I saved it in an Access 2003 format so that I could work on it here at work.
BTW, the reason the form crashed on you is because you had the main form's record source as a query that had many tables rather than just the asset table. You will have to replace a few of your controls with combo boxes (those with name#). You cannot have those as fixed values since the users will have to select the appropriate values when they are adding new records.