Daryan, that device *has* to exist somewhere in the pro audio world. Just not sure if it'll be DC powered
IMO I wouldn't want another signal device in the chain, as I feel every device/cable you add can potentially degrade the signal. I like short and sweet.
As zhianosatch pointed out, I watch levels on my recording device; be it NJB3 or laptop. I prefer the laptop, as it has HUGE level meters, with -db readings, peak hold feature, and is quite accurate. I glance at the screen after every few songs to check the peaks, very convienient.