Ah okay! Thought there was an option to swap them in the ProtoDac v2 with one of the chips mentioned in this threat.
If someone needs a TDA1541 send me a PM 
Hello,
How did you manage to invert the phase in volumio?
Hi Carlos,
I don’t and there is no need to invert the phase. The signals are AC, the phase doesn’t matter.
My tube amp inverts the signal twice with 6SN7, and inverts the signal again with 300B, the final output is 180 deg out of phase. There is nothing wrong with the sound no matter the tube amp is fed with ProtoDAC or other 0 deg DAC such as PCM5242.
P.S. Phase inversion with power transformer is another story due to the internal construction.
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I think the whole “concept” of this dac is to keep it as analoge as possible. No opsmps, no over sampling, no digital filters. The downside of a “passive I/V stage” is that the chip sees resistance on output - thats normally mitigated with an active stage - but running 8 chips in parallel, om top of averaging out variance - mitigate the restance a lot since the load is shared across 8 dac chips. Having just 2 chips sounds like it will give less bass and just less of a lot.
Also, adding passive potentiometer or volume is bad too cause it adds more resistance to a signal that was already seeing the passive IV stage. Use a real preamp.
You’re right. It’s optimal to use a tube preamp in the next stage.