I was looking at the O2 amplifier (not on Massdrop though) and was wondering if I'm using my lgv40 as the source do I need a DAC AMP or will a regular amp better since my phone's output bitrate is 32 bit whereas the DAC AMP is 24 bit also I'm gonna plug it into my wall in my apartment here in the US so 110 volts adapter or 220 volts adapter ? Also since I will be using the 6XX by Massdrop and Sennhiser what gain option should I choose for them 1 and 3..... 1 and 4..... Etc.

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V40 is a solid source, you would need to spend quite a bit on a DAC/Amp to better that plus a quality amp. I own the O2 from MD... was in on the initial drop and it's one of things that brought me here. While it is a very capable amp I've never really fell in love with it. The O2 will certainly make things louder and it is a very clean source so it helps reduce a bit of the sennheiser wool in the bass. I prefer the HD650 out of my iHA-6, H10 and Lyr far more than the O2. The idsd nano black label would be a more interesting and feature rich option IMO but the 6xx/650/600 tend to keep getting better the more you spend on the upstream chain so you will have to find your own "happy place".
There will always be minor sonic differences when hooking different gear together, with quality gear the differences are rarely night and day, more on the subtle side, in my experience. If you have more digital sources you want to connect (PC, Disc Player, Streamer, etc..) you may want an external DAC and amp or an all in one amp/dac. If you exclusively use your V40 and just want more flexibility/power for your headphones then pick up an amp and skip the DAC. Gain: The 1x and 3.3x gain is best if you have easy to power IEM's or earphones. If all you have is full size higher impedance headphones and you listen loudly then go for 2.5 and 6.5x.
Jan 21, 2019