Drop/Hifiman HE5XX (I own two). https://drop.com/buy/drop-hifiman-he5xx

Mogami or Canare quad microphone cable.

I've found Takstar to be a good bang for the buck headphones, if your only paying $66 for headphones (Pro 82 v2), than the FiiO KA13 ($68) is about as much you would want to spend for a DAC/amp to use with them (Pro 82 v2) And it seems the KA13 comes with an adapter, allowing it to be connected...

A headphone amplifier, with a low output impedance might help with a tighter bass. But as your doing audio production, you might invest into some Sony MDR-V6 monitor headphones (used $65?), they should(?) have less biased in the audio. As I would guess the Fostex TH900 MK2 headphone are more...

With 25-Ohm headphones, it's recommended to plug them into a headphone jack with an output impedance of 2.5-(Ohms, or less), for best damping control. Can't find a listing for the output impedance of the Motu M4, so it's output impedance might be well above 2.5-Ohms. OTL tube headphone...

Looks like the only digital output from the M-audio Fast Track Pro is coaxial. So you would need a DAC with S/PDIF coaxial input. But my two cents is to just get a newer Audio Interface. Budget?

Massdrop/Hifiman HD5XX headphones :)

I own the Essence STX (PCI-E), and used aftermarket op-amps. Stopped using it years ago and now stick to USB DACs. I've listened to modern $100 USB DACs and to me they sounded just as good as my STX. So something like a $150 Topping (E30 II) DAC should be pleasing to your ears :)

I would guess a modern $100 USB DAC would equal the Essence ST (PCI) sound card.

They are only 18-Ohms, so should not need much voltage to drive them. So my $25 FiiO battery powered portable headphone amplifier would drive them.

I own the DT880 600-Ohm, slightly bright sound, but other wise good sounding headphones :)

Are those 250-Ohm or 600-Ohm or other?

You mean single channel op-amps vs double channel op-amps? Each slot on the sound card is dual-channel, so it come stock with three dual-channel op-amps. Or you could use 6 single-channel op-amps, in those same three slots. I'm not the expert, but I believe single channel op-amps isolate each...

I've never listened to the Takstar HD6500, but I would expect the Takstar HF 580 (planar) headphones to sound better. (I own the Takstar HD6000 & HF 580). The Drop (Massdrop) Hifiman HD5XX are really good for their $180 price. https://drop.com/buy/drop-hifiman-he5xx

Which Takstar are you asking about? Check out the Takstar HF 580 planar headphones.