What should I upgrade first amp or preamp?

I need your help. I have OPPO BDP-105 connected to PASS Labs XP-10 and XA-30.5 driving 89 dB at 4 Ohm Snell C7. Looking to upgrade my system and planning to spend $ 3000-4000. I live in the apartment building and unfortunately can't listen to music very loud (no more than 58-62 on the XP-10 display).
What would you suggest to do, upgrade preamp to XP-20 first or amp to two XA-60.5 or to XA-100.5 and why. Upgrading an amp is more problematic due to the space constrains.
What will I gain and what should be the difference in sound in each of those cases?
Thank you.
I agree with Zd. Another option would be to send your Oppo to ModWright. I think you will be very pleased with what comes back.
Thank you for your responds. I looked at ModWrite ipgrades befoere, however I'm not a big fan of tubes and prefer SS in all chains.
Why not upgrade power cords and add power conditioner which will be far cheaper than adding new component.
Most high end equipment has the potential of performing much better than what most audiophiles are getting out of their systems. I have done quite a number of experiments over the years using a range of cables, conditioning, tweaks etc. I have been amazed at the capabilities of even fairly low priced gear when the right supporting elements are in place. Sometimes it takes some patience, but in most cases major improvements can be made at very reasonable cost. I agree with Lapierre that keeping what you have and maximizing performance is the most cost effective way of making upgrades. As a starting point you might want to check the Mapleshade web site.
the other option is a new DAC to run your oppo to your pre.
You may also be able to add a couple of power cords and stay in budget.