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

I’d agree with @jhnnrrs about the equalizer you have in the chain.  I’m not sure why you would need it given you have an 8 band in the preamp.  Pull that thing out and get yourself a nice external DAC.  Not sure how much of your listening is digital vice vinyl but an external DAC would likely make a huge difference depending on budget.  You could likely do better for speakers given the level of your other components but I’ve never heard the Spendors.

- Get a Rythmik F12G subwoofer or a pair and cure the Spendor’s flaccidity.

- Get rid of the Panamax and get a Audioquest Powerquest 505, 707 (if on a budget) or something from the Niagara series and plug all your stuff into it. You should be able to perceive more resolution, detail, etc with the latter.

- The room is atleast 50%. Figure out how to treat your room, if you haven’t done so already. If you don’t know much about it, talk to a guy like Matthew Poes (Poes Acoustics). The 150 bucks or so that a knowledgeable acoustics guy may charge for a phone consultation should help you not make mistakes/waste cash and get you far.

Without a picture of your room and equipment layout, we are missing some key info to help you. It is highly probable that the biggest sound improvements you could make are with the room and equipment placement.

I’d start by watching videos on speaker placement  and room treatment. That’s free, and often can make a huge difference.

Second, instead of casting about wildly, test your system and see what is going on.

There is free room acoustic software called  “REW”.  For whatever reason I can’t post a link due to how they set this website up.  But you can run it with a computer, DAC, and a cheap (like $20) omnidirectional microphone.  It will test your system and room acoustics and tell you what is going on.  Very easy to use.  Starting with this software, I literally made a Cold War era bomb shelter into a ln amazing listening space.

Third, Google lists of songs reviewers use to test equipment and what they are used for.  Resolution, bass, separation, etc.  it’s things like “can you hear John’s squeaky chair at the end of a particular Sgt Pepper song?  can you close your eyes and tell the five singers in Y are placed.  Does a Queen song song right?  Does Lourde’s bass in “royals” pound your chest? This is actually fun. And was a fair amount of music I never listen to.

Do all that, and you’ll find your weak link.  If I am guessing, I bet it will be your speakers, which are actually great speakers for their price, but not as nice as your amp, for example.  That said, it may be as simple as buying a subwoofer and adding to those speakers and setting it up with a high pass filter so your main speakers don't have to work so hard.  Perhaps a split before the amp, even.  Just depends on what you buy.

Second guess is the amp, third would be your cartridge.  And that could be how it is installed, which you can also check for free.

What the problem IS NOT are interconnections. That’s an issue for extremely high fidelity systems.  So unless you are using lamp wire,‘don’t concern yourself with that now. Utter waste of money with this system.

it is not your main wires, either.  Farther down the list than interconnects.  It could be (if you hear buzzing or pops or whatever) bad electricity coming in.  A reasonable Furman power conditioner might help and is good insurance from power surges.  This depends on your house and your local power.  But is majoring in minors, typically.

If it were my system, add a stereo pair of subs. Something small and fast like REL T7/x or T9/x depending on size of room. Spend the time learning about positioning and dialing them in properly.