I understand that learning the tones is critical to learning to use this machine successfully. Haven't even had mine out in the field yet but am wondering just how important a good set of headphones is to this detector? I've been using a $20 set of Sony headphones with my ACE 250 and I haven't had any problems with them. Think these will be OK or upgrade to one of the high-end sets? Don't know if it makes any difference but I plan to search using a threshold tone since that's what Garrett recommends. Thanks.