Electronic noise floor to max record/playback level. With +4 levels, you are getting an extra 14db of dynamic range before distortion. Forget all of those obscure things any of you all may be reading. There is a reason that Pro-Recording standard has been +4 for eons. Did any of you know that 3M made a machine that recorded/played back at +13 once? Required special tape. Regular 2" tape would melt on the heads!!! LOL Can't remember who it was that made a machine that was standard at +6. Anyway, more headroom, more dynamic range, better signal to noise ratio. Blah Blah Blah.
Oh, by the way, the Japers also embraced the minidisc system. And invented Kareoke. And Teac (who is the parent company of Tascam. They bought them out years ago so they could have Pro quality gear available) has a vested interest in keeping the waters muddied. They don't want to go the route of retooling the whole thing to accomdate a different scheme.
If you have a digital system with -10 +4 options on the A/D,D/A converters, try them both. Try to tell me that there isn't a significant difference.
Also, there were -10 balanced components. Plenty of +4 unbalanced also.
About introducing noise to your recorder. Emeric is sort of right. If the conversion kit is a step up transformer kit, then the transformers will add distrotion. That could constitute noise I suppose. But as far as adding noise as like in rushing noise (sounds like a waterfall) I don't think that is going to happen. Yes, any noise that is present will be that much louder, but, by contrast, so will the audio signal, so you will retain, if not improve your sound to noise ratio.
Bottom line, if the conversion kit is replacing the OP amps on the recorder, go for it. You WILL be happy that you did.
I will not respond any more here. (Unless something really crazy is said, like Slash is a better guitar player than EVH
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Ed