Hand me down audio equip- upgrade advice

I'd start with at least one if not two dedicated 20 amp circuits with audiophile quality AC duplexes. If you can't do that then at least upgrade your AC outlets.

Wow, what a generous and significant gift! Straight up, with resale value and $1500 put in you are not assured a significantly higher performance from swapping.  A demo comparison would be important.  

You didn't mention cables. If you do not have aftermarket ones, 1500 in cables for entire system would be appropriate and efficacious.  
I have no interest in debating my recommendations.   :)

HI,
Power distribution is as important if you have already solved the signal cabling. If happy with your speakers now why change?

Thank you for your input. Yes, a very generous gift, indeed. I have foundation 
research cables and conditioners for everything- I’ll be updating outlets in the future. I also have good quality cabling for each component- so at this point I’d like to dabble with some different speakers. I don’t intend to get rid of anything from my current setup at this time, just don’t know how much I’m missing out on without trying something more robust. 
I plan on going to some listening rooms, but unfortunately they aren’t in abundance where I live. 

Try and hear a recent iteration of the Vandersteen 2--a used model should fall in your price range.

How's your room acoustics?  Any treatment applied at all?  

Since you have a great sub  Agree that the Vandersteen 2 has great merit.
what a fantastic boss!!!

What are you trying to gain? The only clue so far is "more robust". What do you mean by that?

+1 @millercarbon — We really don’t have much to go on here to make any meaningful recommendations.  What aspects of system and speaker performance are most important to you, and what would you most like to improve upon?  If you’re not sure what those things are yet, you should listen to as many systems as possible to solidify what you really like.  If you don’t have many dealers nearby, you might consider traveling to an audio show or two to see what really knocks your socks off and go from there.  FWIW, and best of luck in your search. 

I will venture a guess that your listening room has no acoustic treatments in it. I say this because most don't not as a slam. If I had the equipment listed, I would use at least half of the $1,500 for room treatments before I even thought about a speaker upgrade from the KEFs. In most set-ups without room treatments, 50%-60% of what you're hearing is the room and not the speakers. Just my $0.02,

That list is hand me downs? For f@$*s sake... Save some more cash and spend a little more on your next towers, the rest of your system can handle it. $3000 to $5000 opens up a lot of possibilities, especially on the used market. The $1500 market is a side step in my opinion. 

I agree with room treatment. I also agree that you have to spend more for a real speaker upgrade. I used to own the KEF 103/3 until I got the bigger and better (although older) 105 which I also restored. My 103/3 had the LF drivers refoamed and the donuts replaced. Fresh ferrofluid for tweeters. Crossover was recapped with Alcap from Falcon Acoustics. The KUBE also got new electrolytics. The 103/3 when properly restored are great speakers. 

@rillerr,   I wasn’t familiar with some of that equipment and went to look it up. That was a very nice gesture for him to give you equipment of that quality and value.  I wish I knew people like that — any boss, friend or acquaintance I have had would gift me an Emerson radio with a cracked case and broken antenna!

Nice gift from "da boss"!  I would suspect he or she is a valuable resource as they owned the equipment and upgraded the whole system.  Maybe you can follow that same road to a lesser degree. Get the room straightened out, that will pay you back with any system you acquire.
Good luck, go slow.

Lots of good advice above.  $1500 doesn't get you in the game these days unless you get lucky, or can fine another mystery benefactor!  I remember the 103/3 w/KUBE without a lot of fondness, so the upside is large if you are willing to re-foam another used pair of classics...find a good pair of Theils, Snells, or a/d/s L1290s if you can, or SF Grand Pianos.  Also, if you are handy with woodworking tools, consider a pair of Dayton Audio Solstice from Parts Express...all Morel drivers.

Thank you all for your comments and suggestions! To answer @three_easy_payments and some others- I don't have any acoustic treatments currently- I'm actually planning on moving in the not too distant future... so much more effort will be put into the room after that happens. 

I didn't know a ton about higher end audio equipment until after I received this equipment as a gift, and after doing a lot of research- I'm very blessed and thankful indeed! 

@barts I have been working with him on plans for an upgrade, but he is currently very far out of my league on everything... He has given me some solid tips and advice though. 

@audite84 and @crustycoot Did you personally refurbish your 103/3's? or did you have someone else do the work? I refoamed the woofers in both speakers, but haven't touched the ferrofluid in the tweeters yet. I've been debating whether or not to do it myself or have a shop do it. I know from what I've read that the tweeters are probably less than ideal as they sit- but I have to say I'm still not convinced they sound bad by any means. I've also had my eye out for older Theils and SF varieties, and will continue to look. I'd love finding an older pair that's not too beat up that I could refurbish.

@guy-incognito I've been having the same inclinations you mentioned, I'll probably just continue to hold out until I feel comfortable spending $5-8k and in the meantime keep an eye out for deals of older proven speakers that I might be able to bring back to life. 

@millercarbon and @soix I see what you’re getting at, part of my issue is definitely the lack of auditioning. I think what I’m after is a more full range sound. The KEF’s really struggle with mid-lower range (I have done different tests on them, but don’t remember the exact roll off struggles). I primarily listen to a lot of jazz, symphonies, 60’s and 70’s light rock, with some hip hop thrown in there occasionally. I’ve been keeping my eye out for a second vandersteen v2w that I could incorporate into the system, but ultimately would like to get a bigger sound stage than my current KEF’s.

Explore planars perhaps—Magnepan/ML.. you don’t pay for expensive boxes ie.good value/sound for the dollar—life-size staging and sounds great generally—should work well with your current rig.

First you need to find out what your boss is going to hand you down next. If nothing, ask your boss what you should get, and pick up some brownie points. Sorry!

You could probably sell the Classe, NuForce, and KEF for about enough to get a Raven Blackhawk and Tekton Double Impacts. Depending on what you can get for them this could be anywhere from a little cash in your pocket to costing about what you're planning now. Only instead of baby steps this would be a big leap forward. Bigger sound stage, plenty of bass, and a rig that will sound great with all your range of music.

OP, 

Someone with better hands did the refoam and ferrofluid. I did the recap of crossover and the Kube. If you have the time and the confidence (well, you did the refoam), I think you can do the ferrofluid as well. 

You have already identified that you like your set up - the Kef is a great speaker, however you want to expand the soundstage. 

In order to maximize your existing set up and achieve a  better soundstage with your max budget of $1,500 the best path forward (as others have pointed out) is investing in upgrading what you already have by:
1. Adding another Vandy Sub.- used - Your adding another L&R diver to the kefs, with a dedicated power source. Expands the Mid/ Low range.
2. Investing in Power delivery. - used, Think of it as another component to your system. Will add detail,Imaging, Depth Texture - maximize your amp preamp performance. 
3. Look into upgrading your crossovers/recap etc.  - Cleaner, efficient presentation. 
And I don't even think your budget is going to cover that, but its a solid move towards getting to be where you want to be.

Investing in different speakers in your budget is a lateral/backwards move that will not solve your issues. You want to make investments that move you forward towards achieving your goals. 

Good Luck

Easy upgrade. I just sold my Monitor Audio silver 300 current model for $1299. They are a fantastic value with excellent bottom end. That would leave a few dollars for a decent set of cables, if needed. I liked them so much I upgraded to the gold 300s. Good luck. Ridiculous $$ spent on electrical solutions are frivolous at your stage of the game. Speakers and DaCS will provide a noteworthy improvement you will always hear the difference every day. 

Build the SEAS Thor kit.   The whole project would probably run you about $2300 when you were done with it.  

https://www.madisoundspeakerstore.com/mtm-speaker-kits/new-thor-kit-pair/

That may seem like a lot, but you'd get a speaker very similar to the Joseph Audio Perspective 2, which retails for $13,000.

http://www.josephaudio.com/perspective

Speaking of A/C circuit upgrades, I found that the 20 amp circuit for my clothes washer works great.  In fact, I moved  my washer and dryer out to the garage to make room for my electronics, and the cost was minimal, due to being able to run the wiring into the adjoining garage.

My speaker cables are still only 12 feet long to the next room, because of my amps' positioning.

After years of incremental upgrades my advice would be to save your money and buy your "dream" system.

 Wait for the move, get to know how your system sounds in the new surroundings, then consider room treatment. In the meantime listen to candidate speakers when opportunity strikes.