xxx1313
500+ Head-Fier
I only wish I could speak Japanese as well as you speak English! Yes, optical is a good option, but most AK DAPs have optical out and since optical is digital, there is no reason why optical from SP1000 would be any better from say an AK70. Another positive about optical is that you can still charge your DAP while playing.
In theory yes, but try yourself. If you do, you might be surprised (at least I was). I personally preferred the optical output of AK380Cu over SP1000SS with Hugo 2. This is not a statement about the quality of their optical outputs, which is good for both DAPs, but their tonality is quite different. SP1000's USB and Toslink sounded the same, imo.