Best Bang for the Buck Upgrade

Biwire?

Tube amp in lieu of hybrid?

It's a 'system' so changing one thing is just a part  of the bang...

Improving your power to the system ( I use and recomend Puritan Audio PSM136 or PSM156) will  upgrade your source and amplification in one swell foop. I have dedicated lines and PSM1136 still made a big difference in timbre, sooundstaging and dynamic impact. As davidten and others recomended, no matter how your audio journey unfolds, getting the power right is foundational.

I knew you were going to say top end and mid bass! It's always the case IDK why, but subs can't fix the midbass and while often recordings are often to blame for the top end I've found after my last round of speakers that a few companies are getting it right. The tweeter in the salon 2 is one, but you would need twice the power for those. The Kef Blades have an amazing tweeter and manage the midbass transition with just a little toe in. I upgraded my speakers after 25 years of being flogged so tbh you will probably get more fun just from moving the system to the long wall and play around. You already have a complimentary system.

Most people still don't understand digital room correction, or will not even try it.

I dare you to put in a DSPeaker Anti-Mode 2.0 room correction system in your already excellent system, run it and set it up properly, use any minor DSP equalization for other peaks as you choose, and then seriously listen to it A/B and blind with someone else running it.

Of course you may be too familiar (and like) the current sound and recognize it. Ask audio folks who don't know the system.

It was the single thing that made my system (and the room) far better, and very audibly easy to hear. I'd never get rid of it.

Unless you have the perfect room and speakers I doubt any single piece of upstream electronics you can buy will make anything close to the positive difference you'll hear with the room correction unit.

Good luck figuring it out and have fun. Congrats on the bonus!

Step 1 - 2 Subwoofers. Properly integrated, subs will improve any system.

I would suggest the REL T series. Use the high level inputs from your amp for easy integration.

Will leave you plenty of money left over for Step 2, a new DAC/Streamer.