Output from Mola Mola preamp to Headphone Amp?

Most headphone amps uses an analog volume potentiometer or stepped attenuator up front, to pre-attenuate the input signal before it hits any active circuitry. Proper protocol is to turn this volume down when headphones are not in use. That makes it safe no matter what signal is coming in (also you are less likely to blast yourself when plugging in).

Even if that’s not the case, you’d be extremely unlikely to "fry" anything in the headphone amp, with the Makua’s maximum output of just 7.75 Volts - quite frankly any circuit which gets fried by that level of input is a bad design, with some modern balanced DACs now outputting up to 10 Volts! That said, it would be bad form to run like this without at least turning the headphone amp’s volume down. If you leave the headphone amp on and volume control up with headphones connected in such a scenario, it would be possible to fry the divers - but I’m not sure how you could ignore the horrible noise!

A few headphone amps do not pre-attenuate the signal, and in that case they should list a "maximum input level" specification. You’d want to check that, and heed it. If it’s listed as "Infinite", then that means the analog volume control comes first in the circuit, and "volume down" is the way to go for safety.

If your sources have multiple outputs, such as XLR and RCA, then use 1 output to the preamp and another to the headphone amp. I do this with my Schitt Ygg+ DACs and my Benchmark DAC3B.

I also do this with my Magnum Dynalab MD108T tuner. 

Some gear does not support having 2 outputs connected at the same time so check with the company that created the source unit.

To answer your question, the answer is NO.  The Makua has to sets of outputs, so if you plug your main amplifier into one and the headphone amplifier into the other, you will get no sound to either with the volume set to zero.  You need a preamp with a tape or line out (fixed) to plug into your headphone amp, that way you can turn the volume down on the preamp and still feed the headphone amp.  Otherwise, if you plug into the outputs, the volume must be turned up to get headphones and then sound would be coming out your speakers as well.  So the quick fix to that is to simply power off your main amp, set your volume at a good setting not to overdrive your headphone amplifier.  Better yet, just plug the headphone amplifier into your source instead if able.

Thanks all for the pragmatic advice. @hifihandyman problem is I don’t want to be connecting and disconnecting my source all the time - cable termination wear and tear paranoia on expensive gear and cables etc…

Mola Mola also got back to me and said it should be okay, provided impedances are compatible and that I only ever use the Manuka volume knob and leave the headphone amp set to 0.

But this seems less than ideal and they did say the output from the Makua wouldn’t be typical for the input of the headphone amp, so I’d wonder what effect that might have on sound. Looks like I either need to give up my Mola Mola Makua with embedded Tambaqui for a preamp with a tape/line out, or I need to settling for a new and secondary source chain for my future headphone amp, which won’t be anywhere near as high end as what I have in my main system. Seems like the more practical option is to replace the Makua and Tambaqui, problem is I love the sound of both and I don’t want to seperate the DAC from the preamp.

Still in the early planning phase of setting my direction for my headphone setup so this gives me a leading direction to explore. Thanks all.

I solved this problem by having separate headphone system and main system. Certainly more expensive but incredibly flexible and you can optimize the sound for headphones.