How big a change would this upgrade make?

In my experience at the level you now have, great improvements are unlikely. As to which I would upgrade it would be the pre amp or the DAC. The new PS audio DAC has some very nice reviews. Most of all do you really like your speakers? They make the biggest difference over any good component IMO. Happy hunting and listening.

Wow! That would be an awesome system!
The reviewer of the ARC 75 (TAS? Stereophile?) claimed
this amp is unstoppable (seemingly unlimited power).
I'd be sure to audition with your speakers first.
Then try with Revel 208's or Bryston Model T.
Don't be afraid to crank it up as this is why you're
paying the big bucks (grace under pressure).
Keep us informed...

Most of all do you really like your speakers? They make the biggest difference over any good component IMO.

The speakers are great; I'm not contemplating a change there. Specifically, my listening room is better suited for monitors than floorstanders (12.5" front to back). I'm not sure I can find better monitors than the Pulsars even at a much higher price.

Looking at what you've got Rrolack, I hope you tried direct connection between The PS Audio Dac and your X30.5 poweramp as Paul McGowin has said the dac has what he calls "Resolution perfect volume and balance controls". Maybe upgrade this to DirectStream.

The output impedance of this PS Audio dac at 100ohms is able to drive the lowish input impedance of the XA30.5 far better than either pre, it's much lower and stronger than the ARC preamps at 850ohms and 300ohms.
And it has a switchable in output level of 1.4v or 2.8v
Try the 1.8v first and if loud enough up to full on the dac's volume control leave it there if not switch to the 2.8v

This will give you the most transparent and dynamic way to transfer the music of the source to the poweramp/s, and if as good as I think it will be you can make money out of it with the sale of a preamp.

Remember what Nelson Pass said about removing the preamp.

A Quote from the master Nelson Pass

Nelson Pass,
"We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.
Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.
Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.
What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.
And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp."

Cheers George

Get a record player. I have nothing against digital and I play plenty of CDs. But IMO,if you respond to vinyl, it will bring you more enjoyment than more expensive amps. Look into it. My suggestion is - instead of buying 11 grand worth of stuff, buy a Music Hall MMF7 for $1400 or so and try it. If you don't like it, you can sell it for $800 or so and move on with your life. It's a lot smaller gamble than what you're thinking about. Yes, you'll need a phono preamp, so add $400 or so, but you're still under 2 grand for the whole thing.