I'm sorry, but this is simply wrong.
More voltage (or power) will NOT make things sound better, it ONLY makes things louder. There might be reasons that one amplifier sounds better than another, but those reasons are
not due to power or voltage. Quality of power is much more important than quantity of power.
The HD 6XX/650 will go beyond 120dB with 8V. 120dB is loud enough damage your hearing within seconds. Listening at a lower volume means less voltage is used and anything beyond that is essentially wasted. 15V would destroy your ears and then it would probably destroy your headphones.
300mW will also push the HD 6XX/650 beyond 120dB.
Are there better amplifiers than the Little Dot? Perhaps. Probably. I've never heard the Little Dot, so I can't say. But I do know that you don't need more power than it can provide.
I don't generally point this stuff out because I don't want to get drawn into big, drawn-out discussions or, worse, an argument. Also, I don't want to pose as an authority where I'm not. Undoubtedly there are also people that are more knowledgeable about it than I am— hopefully some of them will drop in to help clarify.
Anyway, to
@Flex2d: I have the HD 650 and the DT 1990 and I find the DT 1990 to be noticeably better in almost all regards, except perhaps soundstage, which sounds about the same to me. It also doesn't have the beautiful mids-forward presentation of the HD 650, but the mids are still very pleasant and present. A final caveat for the DT 1990 is that the treble is a bit hot, which you may not like if you're treble averse. I think most in the 1990 thread would agree that the treble is bright but generally not piercing or bothersome. It's a really great headphone.