A discrete "dirt enhancement" device that could be considered an overdrive or just a gritty boost pedal that won't alter your original tone but give you tube-like breakup with some natural compression. Available at JetCityMusic.com.
http://www.jetcitymusic.com/index.php?page=shop.product_details&flypage=vmj_naru.tpl&product_id=607&category_id=31&manufacturer_id=19&option=com_virtuemart&Itemid=89