Don't delete the filters, I need them for reuse:

Code:
Ext.ux.grid.GridFilters = Ext.extend(Ext.util.Observable, {
     //...

    /** @private */
    constructor : function (config) {
        config = config || {};
        this.deferredUpdate = new Ext.util.DelayedTask(this.reload, this);
        this.filters = new Ext.util.MixedCollection();
        this.filters.getKey = function (o) {
            return o ? o.dataIndex : null;
        };
        this.addFilters(config.filters);
        //delete config.filters;
        var filters = config.filters;
        Ext.apply(this, config);
        config.filters = filters;
    },