if you had this system, what would you do to make it sound the best you can

I suggest taking some of your upgrade budget and attend an audio show. AXPONA is coming up but there are shows all over the country now. There is no better way to get an idea of how your system stacks up against what is currently available.

I could easily post the same question you have but I don't need to. I'm running a late '90's vintage system with Krell amp and preamp and Thiel CS6 speakers. When they were new they were solid Class A. I've been to three audio shows (AXPONA, Florida, PNW) and I found out that my system holds its own against most of the six-figure systems I heard. In many cases my setup sounds clearly superior to systems where the cabling and power conditioning cost more than my entire rig.

You will hear lots of speakers and some of them will tickle your fancy (I think the rest of your system is fine). You will see that high end speakers sound REALLY different from each other and what you like may be the opposite of what someone else likes. At the 2018 AXPONA they had a pair of large Wilson speakers in a well appointed room and I thought they sounded like fingernails on a chalkboard. Other people I talked to thought they were the best sound at the show. Once you go through this process you can narrow down the choices and make an informed decision. And besides, it's just plain fun.

8th note agree 100%. Also I would like to add cabling make a difference but you have to experiment yourself.

I would certainly agree with upgrading from the Spendor A5s (48 Hz to 20 kHz) to a pair of floorstanders that offer great SQ of a more extensive frequency range, reaching down to at least 30 Hz (-3dB). There are many affordable options available that won’t break the bank. If a floorstander only reaches the low registers around 50 Hz, one might question its purpose. The Halfman iron rule suggests that a larger cabinet could allow for a fuller and more enjoyable sound across the frequency spectrum. So, why not opt for a floorstander that offers a more comprehensive range? Your electronics are fully capable of supporting it.

You have a nice system with very high quality parts. Congratulations!

Some good advice so far, and no one but you can know what is going to make a difference, but as a start:

Try eliminating the power conditioner entirely. You may not need it, and they often degrade the sound. Give it a try and listen.

Try eliminating the equalizer. If you have used it to "tune" the room, taking it our will make a huge audible difference, but listen for increased spaciousness, air, soundstage improvements. Maybe you don't need it. The SS3IC is incredibly complex and all those sliders and switches are doing something even if they're off, or centered, and maybe not so good. But it's up to you to decide!

You didn't mention room treatment. I've spent thousands on treatment and it made a much bigger difference, improvement, than any component I changed. Ever. I hired someone. Room treatment can really turn into a "parts cannon" project and sometimes does as much harm as good.

Have fun! Don't succumb to "audiophilia nervosa". Good luck!

(Nice RtoR, btw. I have a TEAC A3300SX, Great fun.)

I’m going to second the comments of onhwy61, baylinor, & fredapplegate.
You’ve got good stuff to enjoy music in ways too few do (see tons of folks listening to ‘music’ on the onboard speakers of a mobile device…sweet Martha…😖)
Avoid the pitfalls of perfection-seeking, overanalysis-of-minutiae, and audio nervosa (all of which completely remove the listener from actually enjoying music), give thoughtful and reasonable attention to the possibilities of room-acoustics-treatment (this is a HUGE player in the optimized-audio-game…ya can’t put a silk hat on a pig…if a room is acoustically bad, there is a bit that can be done, but it feels futile at times because a bad room is just a bad room) and…
…enjoy music!