As Keith mentioned, pro audio interfaces. But also something as simple as a ASUS sound card. Buy something used to try out the process maybe, lots of 2i4s on eBay.EQ three channels, effectively creating a crossover for the subwoofer. This requires three channels of amplification which wouldn't be a problem because I have the Topping PA5 II and most subwoofers have amplifiers built in. Where the problem comes is that there aren't too many multi-channel DACs on the market, and those that do exist are expensive.
If you're committed to keeping the gear you have then this is the best place to start. You'll hook it up, fiddle with it, measure it, you might get lucky and like the sound you get. If not move forward, you'll have learned your way around REW while your at it.EQ two channels and try to balance the sub and bookshelf speakers that way. I can do this with the existing setup, using the SE RCA outputs from the SMSL DAC and fiddling with the crossover in whatever sub I get. Maybe less optimal that EQing three channels separately, but no additional cost or swapping out of components
Again, if you're keeping your gear then the miniDSP 2x4 is your best option for sub integration. You can still use your headphone amp. Look into the app SoundSource, it allows you to put PEQ or even FIR on any individual audio device connected to your computer like your DAC.EQ external to the source
- Often allows EQ for more than three channels, examples include the miniDSP 2x4 mentioned earlier, as well as the miniDSP Flex.
- With both of the miniDSP options I'll loose the headphone amplifier functionality that I have
I'd still recommend something in the WiiM offerings over the forthcoming Topping. WiiM has just really nailed the UI and the firmware updates keep bringing new features. Just like hi-rez audio purchases are not necessary - same with balanced connections unless you're in an environment subject to noise. Single ended is fine unless you have a ground loop. I was obsessed with DA-AD-DA conversions and sample rate when I started out too with my first miniDSP - ultimately it matters far far less than a well integrated system and even on the best systems you'd not notice the extra bit of conversion.
You just put up your post with the measurements as I'm typing - only remove peaks and stop at 600hz. It'll sound better, trust me.