Do you need passive speakers and an amp? Would powered speakers be ok? They will save you some desk space. I think we can pull this off and get you a good headphone amp upgrade at the same time for your budget.
I’m thinking a pair of JBL 305PmkII speakers ($300):
I recommend Schiit’s entry level Modi 3+ & Magni 3+ for dac, headphone amp, and speaker preamp ($200 total). If DSD decoding is important to you, the Modi 3+ can be switched out for a Topping E30 or a SMSL Sanksrit 10th Anniversary Edition.
The Magni 3+ is the important piece here, I think. You’ll use its preamp output to control the volume of the JBLs and the RSL. Split the Magni’s preamp output with a pair of these:
Get 2 RCA-to-quarter inch signal cables (does not have to be 10’ long, get the length you need, but get this TYPE of thing):
And 2 standard stereo RCA cables, one rather short, one longer.
Your connections will go like this:
Computer -> Modi 3+ via USB -> Magni 3+ input by short RCA -> split Magni 3+ output, then…
use longer stereo RCA cable to send one L signal and one R signal to the Speedwoofer’s stereo RCA LR input.
use the 2 mono RCA-to-1/4" cables to send one of the split L signals to the left JBL and one of the split R signals to the right JBL.
The JBLs and RSL all have individual volume controls and the RSL has a low pass filter selector on it so you can get everything humming along nicely together. Once you have the speakers and sub volumes set correctly relative to each other, the Magni will become the overall volume control.
When you want headphones, plug 'em into the Magni, and the speakers and sub will mute. Pull out the headphones, Magni controls speaker volume. All this make sense?