Little pieces of this puzzle keep coming in. I can’t afford and don’t have the equipment to drive a subwoofer...but would a more full range (i.e. floor standing) speaker be a better choice when I do decide to upgrade next?
When the subject is cost-effectiveness then:
Skip the filter, waste of money, won't improve the sound in the least. The clamp will, but only if you get a good one.
Always in my experience the most cost effective upgrades are tweaks. By tweaks I mean attending to every single facet and component of the whole entire system. Things like Orange fuse, Cable Elevators, HFT, ECT, PHT will bring improvement far beyond anything you can get for the same money the usual way.
You could for example replace fuses in three components with SR Orange, add Cable Elevators, and a full compliment of HFT, ECT and PHT all for about $2k and no way no how will you ever find any turntable or cartridge or anything else for $2k that will come anywhere close.
Then factor in that these are all "lifetime" in that they will all work wherever you go and whatever you buy forever. Its no contest. Nothing else even comes close.
Also power cords, interconnects, speaker cables, Cones, and Shelf. Same thing. You simply cannot find any component anywhere for say $500 more money that will do what a $500 power cord (or interconnect) (or speaker cable) will do for what you already have. Well, from Synergistic, I should hasten to add. Not that there aren't others this won't work with. I just happen to have 30 years experience that tells me with SR you can throw a dart. Everyone else you pays your money....
Note: not idle speculation- I have TRIED! Compared. Home auditioned. Put a $300 Synergistic Master Coupler on a phono stage, you won't get another phono stage that good (without the pc) until you're into it for three to five times as much. If even then.
My system is heavily tweaked out with all this and more. I've done demo's and removing even one or two of these people notice right away. Heaven forbid I should remove it all, let you hear what the same components sound like the way most people do. Shudder. Cue Brando: "The horror... the horror..."