Imagine being able to fill a pool up with the liquefied sound of a favorite albumsay Miles Davis's mind-expanding masterwork Bitches Brewand then dive in. Sound fun? Lemme tell you: It is. Audiophiles and hi-fi fanatics everywhere are giddily engaging in a similar sort of sensory augmentation via a pair of revolutionary new custom earphones called Ultimate Ears. The base modelbut, trust me, more than adequateUE-5c ($550, www.ultimateears.com) is tuned specifically to optimize the sound of mastered music and has a low-frequency and high-frequency speaker in each ear, delivering an unbelievable salience of soundnot to mention the full spectrum of a recording. You might as well have your head stuck in an amp. Think of something you know by heart, then think again. Most likely, you haven't heard the half of it. (For bigger spenders there's the $900 top-of-the-line UE-10 Pro.) Here's the deal: After a quick visit to an audiologist (included in the price), silicone casts of your inner ears will be on their way to becoming a new pair of nonpareil earphones that fit you and you alone. Turnaround time is about a week, and when you screw these rugged yet comfortable little numbers into your head, you'll finally hear your tunes as the musicians and sound engineers intended. Total immersion, indeed.