How to Connect Headphones to a SMSL DAC

This has gotten more complicated than I anticipated. Perhaps the best thing you can do is use RCA splitters to drive your B&O speakers off one of your DAC’s outputs and the drive your subs off the other set of RCA outs. Then you can run the balanced outputs from the DAC into a dedicated balanced headphone amp of your choosing — one that doesn’t have a preamp/volume control — then you can use the DAC to control everything. As far as a balanced headphone cable, I’d recommend this one with the AECO XLR plugs as it’s a great performer and a great value for what you get. Hope this helps, and best of luck.

https://www.lavricables.com/cables/reference-silver-headphone-upgrade-cable/

Well, I wish I was aware of the rest of your setup going into this because it changes a lot. Here’s what I’d do if I was you. If I’m listening to home audio just leave things as they are if it’s working for you. To switch to headphones just unplug the cables that go into the subs and plug them into the Singxer and switch the DAC from variable to fixed output (so only the Singxer’s volume control is operative). And if you go this route use a good balanced headphone cable into your headphones if you don’t already have one to preserve the balanced connection. It’s not the most elegant or seamless solution, but it’s not really hard to do and it’ll work and you’ll get all the goodness of the SA1 (and your DAC) into your ‘phones. I have a balanced headphone cable recommendation if you’re interested. Sorry for the convoluted answer (it sounds worse in words than it would be in practice and would only take a minute to change over), but I didn’t know all the other stuff you had going on here. You’ve got good headphones and they’ll reward you for the effort, and if you decide to upgrade headphones in the future your electronics will scale up nicely and let them sing.

Yes, I’d use the balanced out from the DAC to the balanced input on the Singxer as they’re both fully balanced.  You’ll want to use the fixed output on the DAC (that disables its volume control) into the amp and just use the Singxer to control the volume.  I thought this was a headphone setup so not sure how the subs figure into this — you’d have to provide more info about what you’re doing here. 

I use a Singxer SA-1 amp with my Hifiman Arya Stealths and it sounds fantastic. It’s a fully-balanced Class-A amp that’s nice as I believe your DAC is also fully balanced, and here’s a nice used one you can get for $398 (a $200 discount). Can’t really go wrong with this one.

https://www.usaudiomart.com/details/649999098-sa-1-headphone-class-a-amplifier-pre-amp-excellent-condition/

You could get this and plug the one side into the RCA output of the DAC and plug the two pair on the other end into your speakers and the headphone amp of your choice.  It’s a cheap and effective solution I’d think.  If you have a budget in mind for a headphone amp I can give you some recommendations if you’d like   Hope this helps.

https://www.amazon.com/Arklove-Shielded-Splitter-Connector-Extension/dp/B0BS1K4ZYQ/ref=sr_1_10?crid=3T0O00OO3RU32&keywords=rca%2Bsplitter%2B2%2Bmale%2Bto%2B4%2Bmale&qid=1689801102&sprefix=rca%2Bsplitter%2B2%2Bmale%2Bto%2B4%2Bmale%2Caps%2C2556&sr=8-10&th=1